Our non-GAAP financial measures exclude the following third quarter items, $8.9 million of non-cash stock-based compensation expense, $4.9 million of non-cash amortization expense of acquired intangibles and debt issuance costs and $2.6 million of transaction-related expenses associated with our acquisition of MetaPack. Our non-GAAP financial measures include a $2.8 million non-GAAP income tax benefit in the third quarter. Our mailing and shipping numbers include service revenue, product revenue and insurance revenue and do not include any revenue from customized postage. Additionally, our third quarter 2018 financial results unless otherwise noted, include MetaPack results from August 15 through September 30.

Please see our third quarter 2018 earnings release and 2018 metrics posted on our investor website for reconciliations of our non-GAAP financial measures to the corresponding GAAP measures.

Now, let me hand the call over to Ken.

Kenneth McBride -- Chairman and Chief Executive Officer

Thanks, Suzanne. Thank you for joining us today. Today, we announced our third quarter results, which included a GAAP revenue of $143.5 million which is up 25% year-over-year, non-GAAP adjusted income per fully diluted share of $2.76, which was up 3% year-over-year and non-GAAP adjusted EBITDA of $61 million, which was up 8% year-over-year. We are very pleased with our third quarter financial performance.

Let me now provide a brief update on our recent acquisition of MetaPack. We successfully closed the acquisition on August 15 and we are actively working on integrating the companies and executing on our long-term strategies. As a reminder, MetaPack is a London based software company providing the world's leading multi-carrier enterprise level e-commerce shipping solution. Their API supports over 450 parcel carriers in more than 200 countries around the world and offers a significant number of sophisticated capabilities, including carrier optimization, track and trace, returns and cross-border shipping to name a few. They have over 500 customers, including many of the world's leading e-commerce retailers and brands such as Adidas, ASICS, Fossil, John Lewis, L'Occitane, Marks & Spencer, Speedo, Ted Baker London, The North Face, Timberland, Urban Outfitters, WH Smith, Zulily and many more.

Our strategic rationale for the acquisition remains multifold. First, we expect MetaPack will accelerate our efforts to expand our business internationally. Second, we expect to accelerate MetaPack's efforts to expand in the U.S. Third, we believe our businesses are highly complementary with very little customer overlap. Fourth, MetaPack's preeminent customer list will allow us to strengthen our position with both private and public parcel carriers around the world. And finally, we believe MetaPack will further strengthen our already significant value proposition with the USPS. We remain very excited about the great acquisition, but we do expect that integrating and fully capturing the synergies associated with the acquisition will be a multi-year process.

With that, now let's turn to a more detailed discussion of our mailing and shipping business. Mailing and shipping revenue was $136.5 million in the third quarter, that was up 28% year-over-year. Revenue growth was driven by strong growth in ARPU or average revenue per paid customer, which is driven by continued strong organic growth and contributions from MetaPack. Our total paid customer metric was 732,000. Paid customers were down slightly versus the third quarter of 2017. More modest performance of paid customers in recent quarters is consistent with our strategic shifts to focusing on the acquisition of shippers which are numerically fewer in number, but where each customer has a much higher lifetime value. With this shift in focus, our revenue has been more driven by growth in ARPU than it has been driven by growth in paid customers.

Our average monthly churn rate during the third quarter was 3% and that's in line with the churn rates we've seen over the past several quarters. Average monthly revenue per paid customer or ARPU was $62.14 in the third quarter, that was up 29% versus the third quarter of 2017. The growth in ARPU benefited from continued organic growth in the shipping focused area of our business and the inclusion of MetaPack. Shipping customers generally pay higher subscription fees and small business mailers and we typically also collect additional partner revenue share payments, commissions and transaction fees tied to the packages that we process on behalf of our shippers.

Total third quarter USPS postage printed was 1.5 billion, that was up 8% versus the third quarter of 2017. The total USPS postage printed metric includes both higher growth shipping volume in traditional non-package mail volume, which continues to see a steady decline. Our management team and all of our employees are very proud of the continued financial and business success we generate for our shareholders.

With that, now let me update everyone on some initiatives we're continuing to focus on in 2018. First, we plan to continue to scale the sales and marketing with a focus on acquiring shipping customers. With our focus on shipping over the past several years, we have seen a significant increase in the average lifetime value of a customer that we acquire. With the greater return on investment, we plan to continue to scale our total sales and marketing expense in 2018. A significant focus of our investment is on the acquisition of e-commerce and other high-volume shippers.

Second, we plan to continue to expand the core features and functionality of our shipping solutions. During the third quarter, we unveiled a great new solution, which will allow our customers to begin to use their voice to control their shipping software in our ShippingEasy product. Using an Amazon Alexa device e-commerce sellers can accomplish hands-free shipping tasks with commands to manage orders, ship orders, print labels, print packing slips, purchase postage, check postage balances and several other commands.

ShippingEasy is the first company ever to bring voice controls to e-commerce shippers. We expect the new Amazon Alexa capability to reduce the time and labor required to manage and fulfill orders for our e-commerce shippers. The solution launched earlier in October and we're very excited about rolling it out to the ShippingEasy customer base. During the third quarter, we also continued to support new sales channels, third-party fulfillment providers, marketplaces and e-commerce tools. We also continue to see success with our ShipEngine API, which offers all of the capabilities of our market-leading ShipStation solution in the form of an API available to third parties such as marketplaces and e-commerce tools that are integrating our shipping solution into their own user interfaces. During 2018, we'll continue to develop new innovative features like our new Alexa solution and will continue to add new integrations with selling channels, marketplaces and e-commerce tools and we'll continue developing our end-marketing our ShipEngine API.

Third, we plan to continue developing new add-on features supporting e-commerce customers. During the third quarter, we continued to develop the inventory management and customer marketing solutions. We continue to see good traction with both solutions within our ShippingEasy customer base and we plan to continue to enhance the features in our inventory management and customer marketing solutions. We will continue to market those solutions to new as well as existing customers.

Fourth, we're going to continue focusing on expanding our international solutions and our international marketing. We've begun discussions with MetaPack's management team to drive the anticipated strategic benefits of this acquisition that we just discussed. MetaPack is part of a long-term strategic investment in both e-commerce shipping and international markets and we will be looking to leverage this asset for the long-term benefit of our shareholders. We'll also continue to drive other international shipping initiatives, the initiative called the Global Advantage Program. As you may recall through this program, we offer customers access to discounted USPS international shipping rates through our private label carrier partnerships. The Global Advantage Program allows us to earn incremental revenue on international packages. Also in the international area, we continue to develop partnerships and market our solution in international markets. We've done integrations in the UK, including Magento, BigCommerce, WooCommerce, SquareSpace, OpenCart and PrestaShop in order to support the e-commerce customers there. We've done integrations with Amazon in multiple countries to support that selling channel as well.

During the third quarter, we continued to gain new customers in the UK, Canada and Australia and will continue to ramp up our business development and marketing efforts in those countries throughout 2018. With the MetaPack acquisition and its broad carrier library we will also now begin to look at expansion into additional countries.

With that, now let's turn to a discussion of some recent news involving the USPS. We would like to provide our perspective on five topics that have garnered some attention from our investors. First, the USPS' proposed 2019 price increase. Second, the United States potential withdrawal from the Universal Postal Union or UPU. Third, the recent Office of the Inspector General report entitled Postal Partnerships: The Complex Role of Middlemen and Discounts in the USPS. Fourth, the status of the President's various studies, reports and task forces on the USPS. And finally, the status of negotiations of our agreements with the USPS.

On the first issue of the USPS' proposed 2019 price increases, we're generally pleased with their proposed rate changes. A 10% increase in First Class postage from $0.50 to $0.55 may have some effect on our legacy mailing business, but the rate increase, if approved, should have a very positive effect on the financial health of the USPS. The rates that our customers pay for priority mail are going up on average a very modest 3%, which should be very competitive with 2019 rate increases of UPS and FedEx.

In addition, the rates for priority mail through retail and USPS.com are going up on average nearly 6%, so the average discount that our customers receive for priority mail versus those channels increases from 13% to 15%, thereby increasing the value proposition of using our service.

On the second issue of the United States potential withdraw from the Universal Postal Union or UPU, we would expect the impact to us to be minimal and potentially positive. The UPU effectively sets transfer pricing between the world's postal authorities for packages that are sent between countries.

First and foremost, we would say that in our view the likelihood of a full US withdraw from the UPU is remote. In fact, the UPU has already reacted to the threat by fast-tracking a review of rates and commissioned a report on the subject. The UPU Director General has publicly indicated his support for reforming the UPU's rates and hopes to bring this issue to a vote in April, 2019. In the unlikely event of a full withdraw from the UPU, we would expect the US to then negotiate individual bilateral agreements with other nations. The rates between the USPS and postal authorities of large developed countries that make up the lion's share of the volume are generally already viewed as fair and reasonable. So there is not likely to be any changes to the vast majority of volume between the USPS in other countries.

The focus of the Trump administration threat of withdraw is the large -- is the classification of China as a developing country, which results in large discounts for Chinese companies shipping into the US. China was classified as a developing nation in 1969, when its volume was very small, but now it represents a significant amount of volume coming into the US and in the view of the Trump administration, their classification is no longer valid and their pricing is unfair. We have no volume of packages that are inbound to the US from China. We would note that one potential outcome of the current negotiation process is that China's rates for shipping into the US are increased to a more reasonable level. If such a change were made, it would be very good for our customers. Our customers predominantly ship within the US and if the rate for China's inbound packages are raised to a more fair level, our customers would better be able to compete with Chinese e-commerce merchants, resulting in potential increases of volumes through our systems.

On the third issue of the recent Office of Inspector General or OIG, report, it has not been made public due to USPS concerns with the commercial sensitivity of competitive product pricing strategy in the report. And since we have also not seen the report, it's very difficult for us to comment. That said, based on the title of the report, we would expect that the report is focused on negotiated service agreements or NSAs, that the USPS offers its customers and its partners in the package business. The general goal of these types of the OIG reports is specifically to review whether a program is meeting its legal criteria, which in the case of NSAs, the legal criteria established is that each NSA must cover its costs and must contribute the legally required minimum of 5.5% of institutional costs. As of the last review of the contribution, the contribution was well above this level and accordingly we would have no reason to believe that the USPS is not generally meeting its obligations regarding the NSA program. We would further expect that any commentary in the report on postal partnerships would reflect a very positive effect that partnerships like ours have had on growth in the USPS' package volumes.

On the fourth issue of the President's various studies, reports and task forces on the USPS, there's been a lot of discussion of potential major changes such as privatization of the USPS. But it's important to understand, any such changes would require an act of Congress. Following the June report from the Office of Management and Budget that recommended that the USPS begin to take steps toward privatization congressional opposition to such a plan has increasingly been made clear. The House of Representatives issued a bipartisan resolution with almost half the house declaring opposition to USPS privatization initiative and the Senate, likewise, issued a resolution with a large number of bipartisan members expressing opposition. In addition President Trump formed a three-month task force in April to study the USPS and the report of that task force was completed and delivered to the President but the report has not been made public. And since we have not seen the report, it's very difficult for us to comment. However, implementation of any major recommendations in the report would again require an act of Congress.

On the final issue of the negotiations of important agreements with the USPS, we would note the following general points. Historically, we've had a significant number of agreements with the USPS and we have negotiated those agreements many times. These types of negotiations are very common for us. Negotiations with the USPS often take many months to finalize. The USPS has many competing priorities and the process just takes time. In general, we expect that updates to our agreements with the USPS will continue to reflect the critical role that we play in their e-commerce package business. Including that, we have the largest private sales force focused on driving USPS growth. We process more than one-third of their priority mail volume in the US and working hand-in-hand with the USPS through our partnership, we have contributed to a significant portion of overall USPS e-commerce package growth over the past five years.

With that now let me hand the call over to Jeff for a more detailed discussion of our financial results.

Jeffrey Carberry -- Chief Financial Officer

Thanks, Ken. We'll now review our third quarter 2018 financial results. The discussion of our financial results today includes non-GAAP financial measures. As Suzanne described, a reconciliation of non-GAAP financial measures to the corresponding GAAP measures can be found in our earnings release and in our 2018 metrics on our investor website.

Total revenue was $143.5 million in Q3 and that was up 25% year-over-year versus Q3 of '17. Total revenue excluding MetaPack was $138.3 million in Q3 and that was up 20% year-over-year versus Q3 of '17. The strong growth in revenue in the third quarter was primarily driven by strong growth in our mailing and shipping business, which was offset by a 19% decline in customized postage.

Mailing and shipping revenue was $136.5 million in Q3 and that was up 28% year-over-year versus (ph) Q3 of '17. Mailing and shipping revenue excluding MetaPack was $131.3 million in Q3 and that was up 23% year-over-year versus Q3 of '17. The growth of mailing and shipping revenue was driven by an increase in ARPU, which was primarily driven by organic growth in shipping, combined with contributions from MetaPack. We estimate that revenue derived from our shipping customers as a percentage of total revenue in Q3 was in the mid-70%s (ph) and grew year-over-year in the mid-30%s (ph). We estimate the shipping revenue excluding MetaPack in Q3 was similarly in the mid-70%s (ph) as a percentage of total revenue and grew year-over-year in the high-20%s (ph). We also estimate that our mailing shipping revenue derived from our SOHO mailers as a percentage of total revenue was in the high-teens, and grew year-over-year in the low-single digits.

Mailing and shipping gross margin was 80.5% in Q3 versus 86.9% in Q3 '17. The decrease in gross margins was primarily attributable to the scaling of our international offerings, including the Global Advantage Program, which can have a lower gross margin profile than our other services fee revenue components.

Gross margins were also negatively impacted by the inclusion of MetaPack, which under US GAAP generated a gross margin of approximately 58% for the period August 15 through September 30. We experienced year-over-year increases in our Q3 costs of sales and marketing, R&D and G&A at rates in excess of our revenue growth, primarily related to strategic initiatives -- strategic investments rather, to support the strong growth and innovation in our mailing and shipping business. And due to the inclusion of MetaPack, we would expect to see the absolute dollars invested in sales and marketing, R&D and G&A to be higher in the fourth quarter relative (ph) to the third quarter in order to drive our growth and innovation. Non-GAAP operating income was $59.5 million in Q3 and that was up 8% year-over-year versus Q3 of '17.

Adjusted EBITDA was $61.0 million in Q3 and that was up 8% year-over-year versus Q3 of 2017. Adjusted EBITDA margin was 42.5% in Q3 versus 49.2% in Q3 of '17. The decrease in adjusted EBITDA margin was attributable to lower gross margins associated with the scaling of our international offerings, higher operating expenses associated with our 2018 initiatives as previously discussed, the ongoing accrual of sales taxes and the inclusion of MetaPack which have substantially lower gross and EBITDA margins.

Non-GAAP adjusted income per fully to share was $2.76 in Q3, based on a non-GAAP tax rate of 11% and that was up 3% year-over-year versus $2.68 per share in Q3 of '17 based on a non-GAAP tax rate of 8.5%. Fully diluted shares used in the EPS calculation was $19.0 million for Q3. Q3 '18 benefited from a reduction of our expected tax rate for 2018 from our previously estimated full year tax rate of 18% to a reduced estimated full year tax rate of 15% full year and that was primarily driven by option exercises in the third quarter.

We ended Q3 with $78 million in cash and investments, which was down $205 million from $283 million at the end of Q2 '18. The decrease in cash and investments was primarily driven by the MetaPack acquisition, which was funded entirely from cash, changes in net working capital, foreign exchange driven net equity adjustments related to MetaPack and mandatory debt repayments and share repurchases. And it was all partially offset by a strong operating cash flow and cash from option exercises.

During Q3, we made a required principal repayment of $2.1 million, resulting in total debt under the credit agreement excluding debt issuance costs of $63.9 million. During Q3, the Company repurchased approximately 49,000 shares at a total cost of approximately $12 million. On October 24 of this year, the Board of Directors approved a new share repurchase plan that will take effect upon expiration of the current plan on November 9 (ph). And that plan authorizes the Company to repurchase up to $90 million of stock over the six months following the effective dates.

Now turning to guidance, our updated guidance now includes contributions from MetaPack for the period of August 15 through December 31. We expect fiscal 2018 revenue to be in the range of $550 million to $580 million, which compares to previous guidance of $530 million to $560 million. We expect 2018 revenue to continue to be driven by our continued focus on our e-commerce driven shipping business. In particular, we would expect our shipping revenue growth to be in the high 20s (ph) to low 30s (ph) year-over-year. We would also expect growth in our mailing and shipping revenue derived from our SOHO mailers to continue to grow in the flat to low single-digit range year-over-year.

Finally, we would expect our customized postage revenue to be down year-over-year as we saw higher-than-expected contribution from high volume orders in 2017, that we would not necessarily expect to repeat in 2018. We expect operating expenses to increase in 2018 greater than the rate of revenue growth and we would expect to see the absolute dollars invested in sales and marketing, R&D and G&A to all be higher in Q4 '18 relative to Q1 through Q3 of '18 and that reflects the strategic investments we discussed earlier, as well as inclusion of MetaPack.

We continue to expect fiscal 2018 adjusted EBITDA to be in the range of $245 million to $265 million. This implies both a fourth quarter and full-year adjusted EBITDA margin in the low 40%s, reflecting both higher operating expenses associated with our headcount investments and sales tax expense and the inclusion of MetaPack which has, as we discussed earlier, significantly lower margins. We expect non-GAAP tax expense will be approximately 15% of non-GAAP pre-tax income for 2018 which compares to our previous estimate of 18%. The reduction of our expected tax rate for 2018 was driven by the level of option exercises in the third quarter. Our non-GAAP tax rate was 22% in the first quarter, 16% in the second quarter, and 11% in the third quarter. Our expected non-GAAP tax rate for the fourth quarter is 11% and what that yields on net is an effective 2018 rate of 15% like we discussed.

Our full year 2018 effective tax rate could differ from our current estimates based on a number of factors including the level of option exercises. We expect fully diluted shares to be between $18.9 million and $19.7 million in 2018. We expect fiscal 2018 non-GAAP adjusted income per fully diluted share to be in the range of $10.60 to $11.60 and that compares to previous guidance of $10.15 to $11.15. And finally, we expect capital expenditures to be in the $2 million to $4 million range in 2018.

With that, let me hand the call over to Kyle for some additional comments on our long-term outlook.

Kyle Huebner -- President

Thanks, Jeff. With our increased focus on shipping and e-commerce shipping focused acquisitions, we have achieved a significant transformation in our business over the past five years. Our acquisition in MetaPack not only deepens our focus on e-commerce shipping but also broadens that focus to include significant global opportunities. We expect our focus to continue to be on shipping for the foreseeable future and believe that we are well positioned to capitalize on what we believe to be an attractive global e-commerce shipping trend. e-commerce growth in the US and Europe has been driven by strong secular trends toward greater e-commerce consumption with growth rates running in the mid-teens in both US and Europe, but with European rates exhibiting some greater regional variability. These attractive e-commerce growth rates provide a natural fundamental driver to our growth that we've seen in our business over the past several years.

In addition, we have successfully demonstrated our ability to grow both our shipping related revenue faster than overall e-commerce growth rates over the past five years, as we focused on and invested in shipping. In particular, we've achieved year-over-year organic growth rates and our shipping related revenue in the mid 20% to mid 30% range in each of the last eight quarters compared to the mid-teen growth rates for overall e-commerce. We believe that we will be able to continue to sustain this trend of growing our shipping revenue faster than overall e-commerce over the long term for several reasons.

First, our solutions are suitable for the largest and most sophisticated e-commerce sellers. So we generally attract higher growth e-commerce companies. Second, we are expanding the scope of our offerings and thereby increasing our customer monetization. Third, our multi-carrier platforms allow us to expand outside our traditional core USPS focused business.

MetaPack's recent growth rate has been in the low-to-mid teens. As outlined in the strategic rationale that Ken discussed, we expect that the MetaPack acquisition will accelerate our international initiatives and we expect that we will be able to enhance their growth rate as we realize various synergies over the course of the time. With the inclusion of MetaPack, our portfolio of solutions now account for over 1.5 billion packages shipped annually worldwide. As shipping revenue as a percent of total revenue increases from its current mid-70% level, total revenue will continue to be more directly correlated with our strong performance and fundamentals in shipping.

We believe that global e-commerce trends provide exceptionally attractive fundamental drivers to our domestic and international businesses and believe we will be able to drive strong top line and bottom line performance for the foreseeable future with our portfolio of exceptional shipping platforms and technologies.

And with that, we will open it up for questions.

Questions and Answers:

Operator

Thank you. (Operator Instructions) Our first question comes from George Sutton from Craig-Hallum. Please proceed with your question.

George Sutton -- Craig-Hallum Capital Group -- Analyst

Thank you and I appreciate all the updates. So I'm curious with respect to the OIG and the task force review, generically our belief has been that the value will increasingly be moving to generators of new customers and incremental revenues which in our view would be a benefit to the model that you have now. I'm curious if you can comment on that. Is it your belief that the summation of all of these different reports could end up leading to more value for you versus less that I think most people would assume.

Kenneth McBride -- Chairman and Chief Executive Officer

Yeah, I think that's an accurate assumption. I think when we look at the various reports that are out there, the conversations we've had with folks that are working on these reports, the general view is that the idea of USPS increasingly embracing partnerships like ours makes a ton of sense for them. I think we mentioned the full-blown privatization isn't really something that we see as likely, but I think more and more partnerships with private companies that will allow them to more embrace of pseudo privatization if you will, has been something that we seem to be -- the task force and other members of the community in the USPS would have been focused on.

George Sutton -- Craig-Hallum Capital Group -- Analyst

Understand. And generally, as you look at these regulatory processes and review, has it changed your hiring plans, your partnership focus expansion in any way as you look forward?

Kenneth McBride -- Chairman and Chief Executive Officer

No, I don't think I can say that it has changed anything. As you know this is an area where we have a significant number of very senior people that focus on our relationships with the USPS and in Washington DC in general. And I think that we have a great team on the ground there and we have I think strategically we've really had a very long relationship with USPS and I think we see that continuing. So we really haven't changed in terms of our hiring plans in the USPS world or from a partnership perspective.

Kyle Huebner -- President

I think you see that in Jeff (ph) results as well, right. When you look at the margin degradation sequentially as well as year-over-year, that's a function of investments, principally in headcounts across the board. So I think you also have Ken's qualitative comments, but also empirical results from us that show that we continue to invest heavily in the business to grow for the long run.

Jeffrey Carberry -- Chief Financial Officer

Right. The long-term fundamentals that I talked about in shipping -- e-commerce shipping international all remain attractive. So our predisposition is to increasing our investments and to the degree that we continue to increase our investments and get an ROI on that, that leads to more success for the USPS for their position in the e-commerce shipping world. So I think us increasing our investment has a win-win on both sides.

George Sutton -- Craig-Hallum Capital Group -- Analyst

Okay, great. And then lastly a lot of questions came up, obviously, when you had a new disclosure in your last 10-Q. I wonder if you could just address the logic for having that now and will we get updates regularly when there are additional programs or conclusions to those negotiations? Thanks.

Kenneth McBride -- Chairman and Chief Executive Officer

I think, in general, like I mentioned in the prepared remarks these types of agreements, these types of negotiations are very common for us. We've had -- over the span of 20 years, we've had dozens of these types of negotiations. I think, it's simply -- this time is just an example of us deciding to provide some additional insight into the negotiations. As I mentioned earlier, the negotiation with the USPS take many months to finalize and so the USPS obviously has a lot on their plate. There's a lot of things going on in the USPS world and so we're working with them closely on the negotiation, but it's just going to take some time. So I think we just felt like in terms of the 10-Q that making that update was appropriate at this time.

George Sutton -- Craig-Hallum Capital Group -- Analyst

Understand. Thanks, guys.

Kenneth McBride -- Chairman and Chief Executive Officer

Sure.

Operator

Thank you. Our next question comes from Zach Cummins from B. Riley FBR. Please proceed with your question.

Zach Cummins -- B. Riley FBR -- Analyst

Hi, thank you. Yeah, congratulations on a really strong quarter. I guess -- are there any updates to your expectations in terms of contribution from MetaPack as you head into Q4 or is it still kind of around that range of the $15 million to $20 million that you provided on the last call?

Jeffrey Carberry -- Chief Financial Officer

Yeah, it's really falling in that same range. So we gave a qualitative guidance last quarter, you saw the increase to the top line that was in line with that qualitative guidance last quarter from MetaPack. So I think it's only fair to assume that we'd expect MetaPack to come in in that range as well.

Zach Cummins -- B. Riley FBR -- Analyst

Great, thank you. And then in terms of as you go into this holiday period, there has been some concerns, especially around Amazon lowering their revenue guidance as they head into the holiday period. Just kind of curious of your thoughts as you head into this and overall what are you feeling in terms of shipping volumes and just the overall view as you go into this really important holiday period?

Kenneth McBride -- Chairman and Chief Executive Officer

I think that our perspectives on the business are reflected in our guidance. We tend not to really focus on providing color on the specific underlying factor of particular e-commerce trends that we're seeing. Obviously, I think the numbers imply that we continue to see a robust business for ourselves which is inherently a function of growth in e-commerce. So we don't give specific commentary around package volume growth and things of that nature that may be inherent and maybe what some other people do. But we still feel very confident about Q4 in terms of performance expectations.

Zach Cummins -- B. Riley FBR -- Analyst

Great, that's helpful. And finally, last question for me. In terms of the new buyback program that you just announced. I'm just curious with that in mind, could you list your priorities when it comes to capital allocation? How do you view investing in the business either organically or through M&A versus buying back shares at this point?

Kenneth McBride -- Chairman and Chief Executive Officer

Yeah, I think, we have a significant cash flow in the business and I think we look at all of the above. When it comes to deploying capital, I think you've seen our trend toward looking at M&A as a way to build the business. When we see a strategic acquisition that makes sense we certainly like to have the ability to do that acquisition financially, but I think we've also continued, the Board has continued to show that it's behind repurchase of the shares in a consistent and ongoing manner. So I think in terms of capital allocation as Kyle mentioned, we're continuing to aggressively invest in the business where we're looking at more for additional M&A opportunities and we are continuing to do a buyback of the shares.

Kyle Huebner -- President

Yes, I would just say, I think when we look at it, we don't feel like we're constrained in having to sacrifice one area at the expense of another. As Ken said well, we're investing in the business, we have the cash flow and the debt capacity to pursue acquisitions as they make sense and we have the buyback program in place. So I think we're in a pretty favorable position where we can capitalize on all the opportunities with our capital allocation and we're not necessarily constrained in one particular area. We look at it as investments that generate ROI and that we have the capability to invest in any of the areas, because we have a very strong capital position.

Zach Cummins -- B. Riley FBR -- Analyst

Right. Great. Thank you for taking my questions and congrats again on the strong quarter.

Kenneth McBride -- Chairman and Chief Executive Officer

Thanks.

Jeffrey Carberry -- Chief Financial Officer

Thanks, Zach.

Kyle Huebner -- President

Thank you.

Operator

Thank you. Our next question comes from Allen Klee from Maxim Group. Please proceed with your question.

Allen Klee -- Maxim Group -- Analyst

Good afternoon. Could you give me a little more understanding on where the synergies are going to come from MetaPack?

Kenneth McBride -- Chairman and Chief Executive Officer

Sure. Yeah, I mean I think we went over some of those last quarter, but I think in general, the idea with MetaPack is that it accelerates our efforts to expand internationally. It provides access to the largest carrier library in the world, 450 parcel carriers, over 450. In our current solution before we bought MetaPack was about 40 carriers. So we immediately saw a 10-fold increase in the number of carriers we can support in our products worldwide. In the U.S. you really have USPS, FedEx and UPS that dominate the market, but when you look at international markets and especially in Europe, you need a much broader footprint in terms of capabilities and the number of carriers you support in order to have a complete solution. So in terms of MetaPack, they brought that breadth of carrier library that we're able to then integrate with our e-commerce solutions like ShipStation in order to be able to go to those markets more quickly. I think the other thing that we discussed was, MetaPack really doesn't have penetration in the US right now.

We have a large national sales team and a network and expertise in the US market that will allow us to provide us a solution in the US market and to support their efforts to go to market in the US. So we've already been working on some of our sales and marketing initiatives. We expect to launch some programs in early 2019. So we see that as a potential early win. Now that the sales cycle is long with MetaPack, so we wouldn't see a huge contribution probably happening right away, but we do see that as a big strategic initiative of ours. I think generally speaking the businesses, when you look at them, were just very complementary.

We're a small business, they're very large retailers in enterprise, they're mostly Europe, we're all US prior to the acquisition and so we really fit between the two companies. So the company is really unique asset. No other company in the world has a carrier library anywhere near theirs and so we really saw that as a great opportunity for us to expand our international efforts.

Kyle Huebner -- President

Yeah, I would just add onto that. If you look specifically at ShipStation, they are focused on e-commerce customers and without the MetaPack acquisition, ShipStation would have to go into Europe and develop all these carrier integrations and relationships from scratch. By acquiring MetaPack, those relationships and integrations are there and that allows ShipStation to really focus on growing the business, getting partnerships integration with marketplaces to grow their business because the carrier relationship integration side is there with MetaPack, whereas they would have had to develop that themselves in the absence of MetaPack.

Allen Klee -- Maxim Group -- Analyst

I guess what I was trying to understand is it sounds like most of what you're saying are revenue synergies, but is it possible that the margin impact of this over the long term, you think you can improve the margins also to get them somewhat comparable to where you guys are?

Jeffrey Carberry -- Chief Financial Officer

Yeah. So, Allen, I would certainly expect to see margin improvement relative to their current position for a number of reasons, but yes, I would expect improvement in margins. It's primarily going to come from growth in the top line, not from savings in the bottom line. So what I'm saying is effectively is there is a fair amount of leverage in their current cost structure.

Allen Klee -- Maxim Group -- Analyst

Thank you.

Operator

Thank you. Our next question comes from Darren Aftahi from Roth Capital Partners. Please proceed with your question.

Darren Aftahi -- ROTH Capital Partners -- Analyst

Hi. Thanks for taking my questions. Just a few, if I may. First on, just the cadence with customers, I understand the shipping dynamic and higher ARPU. I am just kind of curious as we go forward, should we assume that number kind of stays flat to decline, like we had in the last two quarters and that perhaps ARPU growth is maybe stronger than people are assuming? And then as it pertains to the ARPU growth in the quarter I think it was 29% and accelerated. I am just curious on a basis point how much of that was a contributor versus MetaPack versus organic? And I have got a couple of follow-ups.

Jeffrey Carberry -- Chief Financial Officer

Yeah, so in terms of the paid customers, we don't give guidance on paid customers. So (inaudible) that now, but I think from a color standpoint, you see mailers which is a very large portion of the paid customer metric, growing low-single digits. ARPU is generally tend to be reasonably flat with the small business customers, the mailers. So I would generally expect to see customers on the mailing inside kind of flattish, maybe slightly down, maybe slightly up, some variability there with seasonal slowness as well or seasonal factors. So I think you're right, that the majority of the growth, obviously in terms of long-term models can be coming from principally shippers who have higher ARPU. So when you look at the relative contributions, the large portion of that is really coming from ARPU growth versus paid customer growth. In terms of the ARPU growth, certainly a portion of that was a contribution from MetaPack. MetaPack was relatively small around $5 million in the quarter, which you see in our filing. So there certainly was a portion of the ARPU growth attributable to MetaPack, but it was a relatively small portion of that growth.

Kyle Huebner -- President

Yeah. I would just add if you look at it, we said that MetaPack has in the ballpark of 500 customers. So if you look at our metrics and you back out the 5 million MetaPack revenue, really the number of customers is the same. The organic ARPU growth was still in the kind of 25% year-over-year range.

Darren Aftahi -- ROTH Capital Partners -- Analyst

That's helpful. Thank you. And then just one last follow-up, kind of two parts. So you've had this under your belt for about three months now, roughly speaking a little bit over that. Ken, what's been going better on the integration and what's been more challenging? And then, as it pertains to MetaPack having kind of an installed base of some larger retailers, how quickly or will we see that kind of presence of a customer, say, a larger retail maybe in the US in the future? Thank you.

Kenneth McBride -- Chairman and Chief Executive Officer

Sure. I think, we're pleased with the progress so far with the integration. We've been working on initiatives we discussed last quarter. One of our senior management members moved to London to help spearhead the integration and we've already begun the process of bringing MetaPack to the US with our sales and marketing teams. We're expecting to launch some programs in that area in early 2019.

We've been working through the technical challenges and the changes that will be necessary to achieve the integration between our e-commerce solutions like ShipStation and MetaPack's carrier API. We've made good progress, but achieving that full integration, leveraging that integration will take some time. We've seen some really good benefits with our relationships with carriers. As we've gone into meet with those carriers and as we began our negotiations with them for additional product and financial support, we've seen a really positive reaction to the acquisition by partners and by the USPS. So net-net, I think we've been pleased with how the integration has gone so far. It's been smooth and we continue to be very excited about the acquisition.

I think in terms of challenges M&A, acquisitions always have challenges and I think the large difference in time zones has been a challenge, holding meetings with them. Generally there is a difference in culture between US and UK employees and we've been working to align the teams on strategy and sales and marketing and development philosophies. But those challenges so far had been reasonable and again, we're pleased with how the integration process has gone. In terms of bringing retailers to the US, I think like I mentioned, we're just at the very beginning of that process. We really haven't started to reach out with our sales team. Like I said, we're planning to launch that program in early 2019. So we really haven't made any headway in terms of bringing the large retailers to the US.

Darren Aftahi -- ROTH Capital Partners -- Analyst

Thank you.

Operator

Thank you. And our next question comes from Tim Klasell from Northland Securities. Please proceed with your question.

Tim Klasell -- Northland Securities -- Analyst

Hey, good evening, guys. My question has to do around the periodic price increases that we see from all the major carriers out there, USPS, FedEx, UPS. With your relationship with the USPS when these prices go up that they have, how does that flow through to you, particularly as we start may be facing a higher inflationary environment, how will that flow through with your contracts with the USPS without getting into specifics? Is it very little, is it very similar impact on your revenues as what they -- how they raise their prices? Thank you.

Kenneth McBride -- Chairman and Chief Executive Officer

So in terms of our impact, we can't get into the details of the contracts due to confidentiality. But generally speaking, we're monetizing postage dollars. So price increases are generally speaking an improvement for us.

Tim Klasell -- Northland Securities -- Analyst

Okay.

Jeffrey Carberry -- Chief Financial Officer

I think the other aspect of that is, while we have relationships with 40 different carriers, our primary relationship is with the USPS. So we look for them to be and hope that they will be very competitive in terms of their rate increases versus UPS and FedEx. I think last year, we saw them increasing by -- it was about 3% or 4% versus a 5% increase from UPS and FedEx. So we saw some additional business coming to USPS with those improved rates. This year we were -- in 2018 we are pleased to see that they're planning to increase the Priority Mail rate by only 3% and we would probably expect that UPS and FedEx when they announce their rate increases will be higher than that traditionally 4% to 5%. So generally when the USPS gains market share, we benefit. And so I think that's the other aspect of how the rates play through in our business.

Kyle Huebner -- President

And just the other thing we mentioned on the call is to the extent that retailer USPS.com have higher price increases than PC Postage then that differential spread increases the value proposition for our customers using our solutions versus the alternative ways to get the postage for packages.

Tim Klasell -- Northland Securities -- Analyst

Great, thank you very much for the color. Appreciate it. Good night.

Kenneth McBride -- Chairman and Chief Executive Officer

Thanks, Tim.

Operator

Thank you. I'm not showing any further questions at this time. I would now like to turn the call back over to Mr. Ken McBride for any further closing remarks.

Kenneth McBride -- Chairman and Chief Executive Officer

Again, thanks for joining us today. If you have follow-up questions, as always, you can contact us through our Investor Relations website at investor.stamps.com or you can contact us via our investor phone line at 310-482-5830. Thanks for joining us.

Operator

Thank you. And ladies and gentlemen thank you for participating in today's conference. This does conclude today's program and you may all disconnect. Everyone have a wonderful day.

Duration: 56 minutes

Call participants:

Suzanne Park -- Senior Director of Finance

Kenneth McBride -- Chairman and Chief Executive Officer

Jeffrey Carberry -- Chief Financial Officer

Kyle Huebner -- President

George Sutton -- Craig-Hallum Capital Group -- Analyst

Zach Cummins -- B. Riley FBR -- Analyst

Allen Klee -- Maxim Group -- Analyst

Darren Aftahi -- ROTH Capital Partners -- Analyst

Tim Klasell -- Northland Securities -- Analyst

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