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Not Upgrading Your Membership

Gold Star Executive membership costs $120 a year while Gold Star membership costs $60 a year, but you might be able to recoup the cost -- and then some -- with the executive membership's 2% cash back annual reward. If you spend more than $250 a month at Costco, your rewards check for the year will be more than $60, so you'll make up the price difference and then some.

Not Taking Advantage of Price Adjustments

If an item you bought goes on sale within 30 days of your purchase, you can get the difference back as a price adjustment. Not taking advantage of this policy is basically throwing money away.

Not Stocking Up on Gift Cards -- for Yourself

Costco sells gift cards at a discount, so it's worth it to purchase them for yourself for restaurants and retailers that you frequent. You're automatically saving when you shop or dine with the discounted gift card.

Not Taking Advantage of the Costco Concierge Service

You already know that Costco has some of the lowest prices on electronics, but you might not know that your tech purchases give you free access to the superstore's Concierge Service. Costco offers free technical support for members, as well as a two-year warranty on many electronics purchases.

Buying Only Brand-Name Items

Don't overlook Kirkland Signature items. For many products, Costco's store brand is comparable to its name-brand counterpart.

Not Paying Attention to Price Signs

You may not realize it, but there's a lot of information about a product hidden in its price sign. Uproxx reports that the following prices and symbols have specific meanings:

• Prices ending in .99 or .98: These products are retail price, and don't provide savings over other store's prices unless bought in bulk.

• Prices ending in .89, .79, .69, .59, .49, .39, .29, .19 or .09: These are manufacturer deals, so you're getting a lower price than you would elsewhere.

• Prices ending in .97: These are items that are on sale because the store manager needs to move stock.

• Prices ending in .88 or .00: This indicates a steeply discounted product.

• Asterisk: Price signs marked with an asterisk in the top right corner indicate that it's heavily discounted and will be discontinued, so now is the time to stock up.

Avoiding Costco If You're Not a Member

There are some Costo perks even nonmembers can take advantage of. This includes filling prescriptions at the pharmacy, which often has lower prices for drugs than elsewhere, CNBC reported. Nonmembers can also get flu shots and eye exams at Costco, purchase food at the food court and -- in certain states -- buy alcohol at Costco's low prices.

Mistakes To Avoid When Shopping on Amazon

If there's something you want or need, there's a good chance you can find it on Amazon. However, you might be paying too much for your purchases if you're making any of these online shopping errors.

Not Knowing When the Popular Sale Seasons Are

"It's really easy to see something you like and purchase it on the spot -- especially when Amazon makes checking out very convenient -- but what if you knew it went on sale less than a month later?" Gramuglia said. "Seasonal items usually get marked down around the same time every year and sale events like Amazon's Prime Day have similar items discounted every year. Typically, on Prime Day, you can expect prices to be at their lowest amount, even beating Black Friday on popular items from beauty, tech, home, kitchen and virtually every other category you can think of."

Gramuglia notes that although Prime Day traditionally takes place during the first few weeks of July, there are reports that it may be moved to September this year.

Not Subscribing for Household Items You Buy Regularly

You're missing out on a substantial discount on items you buy often if you don't use this feature.

"With Amazon's Subscribe & Save, you can save time and money on everyday items," Gramulgia said. "From detergent to toilet paper, you can subscribe to receive shipments of the items you need on a regular basis for less. And for every five or more products in one auto-delivery, you can get as much as 15% or 20% off if you're a Prime member on subscriptions to diapers, baby food and more."

Not Opting for Free No-Rush Shipping

"If you're an Amazon Prime member and don't need your order right away, consider choosing free no-rush shipping instead of free two-day shipping to earn rewards on future Amazon purchases," Gramulgia said. "This is a great way to save money in the long run."

Not Using an Amazon Rewards Visa Signature Card

"The Amazon Rewards Visa Signature Card or Amazon Prime Rewards Visa Signature Card is a must-have for any Amazon shopper," Gramuglia said. "For every eligible purchase you make with an Amazon Rewards Visa Signature Card, you earn 3% back -- or 5% back if you're a Prime member with the Prime Rewards card -- at Amazon.com and Whole Foods; 2% back at restaurants, gas stations and drugstores; and 1% back everywhere else."

Skipping 'Today's Deals'

You can find Amazon's best deals and steals for new products in the "Today's Deals" link at the top of any Amazon page.

"There you'll be able to shop Amazon's Deal of the Day and Lightning Deals -- the latter is when you only get hours or just minutes to purchase a product on sale," Gramuglia said. "This added browsing can score you more savings than you expect at Amazon."

Assuming Amazon Is a One-Stop Shop

Although it may be convenient to buy everything on Amazon, it doesn't always have the best prices.

"There are plenty of other online retailers that can give you a better price and discount," Gramuglia said. "Before you check out, browse other major online retailers and compare prices to ensure you're really getting the best deal."

Setting Up the One-Click Payment Option

"Yes, it makes things so much easier to buy an item with just one click, but it also makes it much easier to overspend," said Julie Ramhold, consumer analyst with DealNews.com. "Ignore the impulse to make shopping too easy, and leave the one-click settings turned off. If you have to go through the entire process of adding the item to your cart, and choosing shipping, address and payment options, you'll be less likely to buy something you don't need."

Not Comparing Prices for the Same Item in Different Colors

Amazon will sometimes have different prices for the same item in different colors based on the supply and demand for each color, CNBC reported. If the color doesn't matter to you, click through the different options to find the lowest price.

Ignoring Prices From Other Sellers

When you search for an item on Amazon, you don't always see the best price for it on the main product page. On the right column of the product page, you will see a list of "other sellers on Amazon" and the prices they are selling the item for. In some cases, there are lower prices available.

Not Checking the Coupon Box

Applying coupons on Amazon is as easy as checking a box underneath where the item price is listed -- but if you don't look out for it, you could miss it and miss out on the savings.

Not Tracking Prices

Prices on Amazon fluctuate often, so it's worth checking a price tracking site like CamelCamelCamel.com to see if the current price for an item you want is high or low. If you're not in a rush to buy it, you can set an alert that will let you know when the price has dropped.

Mistakes To Avoid When Shopping at Target

With great quality items at fair prices, it's hard not to love shopping at Target. But you could be spending too much money at the beloved retailer if you're making any of these mistakes.

Not Using Your Target RedCard

"If you love a bargain and receiving rewards simply for the shopping you would've done anyway, consider opening a credit card -- or even a debit card -- at Target," Gramuglia said. "The Target RedCard gives users a 5% discount on all purchases, free shipping online and access to RedCard Exclusives including special items, gifts and promotions."

You can even save 5% at any in-store Starbucks location when you use your RedCard, Gramuglia said.

Not Taking Advantage of Target App Coupons

"If you haven't already, download the Target app to gain exclusive access to deals and coupons, browse weekly ads and place orders with Drive Up," Gramuglia said. "It's easier than ever to use coupons to save money and time when shopping through the Target app."

Not Stacking Coupons

"At Target, you can use several coupons per transaction unless the coupons indicate otherwise," said Trae Bodge, smart shopping expert for TrueTrae.com. "You can even stack manufacturers' coupons with Target coupons. Look for stackable coupons on Target.com, your local circular and on a deal site like Slickdeals.net, which has a page dedicated to Target coupons."

Purchasing Items Right Away

You can sometimes save money by abandoning your online shopping cart.

"Stores like Target track our every move while browsing the internet, so when we leave a store's page without making a purchase, they take note of it," Gramuglia said. "If you wait a few days or so, you may receive a 'We miss you!' or 'You forgot something!' email from that store featuring a product you clicked on but didn't buy, and sometimes a customized discount code, too."

Skipping Over the Endcaps

There's no one clearance section at Target, so it's important to not overlook the clearance areas in the respective sections of the store.

"Target scatters their deals and steals throughout the store," Gramuglia said. "And don't ignore the endcaps! You can find some serious savings there."

Not Comparison Shopping

As with Amazon, you may be tempted to purchase everything you need during a single Target shopping trip -- but Target doesn't always have the lowest prices.

"You can likely save money simply by comparing your options," Gramuglia said. "Before you purchase an item, check to see if the price is lower at another store, or if another store is having a sale."

Not Planning Your Trip

As is the case with Costco, you should never go to Target without a shopping list.

"Target is one of those stores that's really easy to walk out of with more than you intended -- there's even a term for it: the 'Target Effect,'" Ramhold said. "A good way to combat this is to plan your trips in minute detail. It sounds overwhelming, but the app can make things much easier as it has a map function so you can pinpoint where the items on your list are. Make your list in the app, note where each item is, and then stick to that list to keep from walking out with five new throw pillows you don't have room for."

Not Utilizing the Option To Pick Up In-Store

Not everything on the Target app and site is available for in-store pickup, but if you only need a few things and they are available for pickup or drive up, consider using this functionality.

"If you opt for store pickup, you'll go inside and go straight to the service desk to pick up your order when it's ready, and if you opt for drive-up service, they'll bring your order out to you," Ramhold said. "This means you won't even have a chance to be enticed by the cute and useful things in the Dollar Spot, or the selection of snacks up front."

Mistakes To Avoid When Shopping at Walmart

Walmart is known for its 'everyday low prices' -- but you still might be spending more than you need to be if you're making these common Walmart shopping mistakes.

Important To Know: 25 Best and Worst Deals at Walmart

Buying Overpriced Items Through a Third-Party Seller

Like Amazon, Walmart.com sells items from third-party sellers.

"Before you buy something on Walmart.com, do a Google search to see if you are getting the lowest price," Gramuglia said. "For example, a pair of Nike sneakers from a third-party seller on Walmart ranges from $89.22 to $98.52, whereas you can buy the exact same shoe at Dick's Sporting Goods for $54.99. Keep in mind that Walmart does not price match for third-party sellers, so comparing prices prior to purchasing is a must."

Buying Something Just Because the Price Seems Good

"Walmart is known for its prices, but whether you are shopping something full-price or on sale at a Rollback price, be sure to always comparison shop to make sure you're actually getting the best deal," Gramuglia said. "Walmart does price-match, but it needs to be the exact item and must apply to other limitations the store sets."

You should also take quality into consideration before making a purchase.

"If you find yourself eyeing a product, like a sweater for $10, consider the quality, style, and longevity and usage of the product," Gramuglia said. "Sometimes inexpensive products end up costing you more in replacing the item than it would if you bought a better quality product. However, you can also find quality products at a lower price, so be sure to weigh the pros and cons to make the best decision for your finances."

Not Double-Checking the Price

"You may find a clearance item or full-priced item that you've been eyeing, but may want to put it back on the shelf because it's out of your budget. But before you do that, double-check the price," Gramuglia said. "Especially since items can be easily moved throughout the large store, always price check your items for a final verdict. Stores like Walmart are constantly running promotions and have items marked down, and may be running a deal that you don't know about that can save you an extra few bucks. The price scanner will give you the most up-to-date cost."

Shopping Too Late in the Day

The early bird gets the best deals at Walmart.

"Making the trip before 9 a.m. is a good idea since that is when everything has been neatly restocked and you have a better chance of getting what you want," said Freya Kuka, founder of the personal finance blog Collecting Cents. "If you are a morning person, this is a great way to start the day."

Overlooking Store-Brand Items

"Walmart has 'Great Value' options that are branded replacements; [opting for these] can help you save hundreds every month," Kuka said. "In most cases, the brand is nothing but a name in comparison."

Shopping for Seasonal Items Too Early

"Usually a month before the next major holiday rolls around, Walmart devotes at least one entire aisle to items for it. And yes, it can be tempting to grab a giant bag of Halloween-themed chocolate in mid-September, but if you wait until the day after the holiday, you'll see incredible discounts that make it so much easier to get a bigger bang for your buck," Ramhold said. "Granted, for things like that, you might need themed candies and items before the holiday itself, but we recommend getting only what you need ahead of time, and then treating yourself to goodies the day after the holiday."

Thinking 'Rollback' Prices Are the Best Deal You'll Get on an Item

"Rollback prices typically last around 90 days, but they aren't actually sale or clearance prices -- they're just a slight price drop due to something like overstock," Ramhold said.

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This article originally appeared on GOBankingRates.com: The 37 Mistakes We Make When Shopping at Costco, Amazon, Target and Walmart