A big reason so many people are willing to pay for a Costco membership is that they make back their fee in the form of savings on the items they buy in the store. You might, for example, spend $65 a year for Costco's basic membership, which is about $5.42 per month. But if you can save $6 on milk and eggs every week by getting them at Costco, then you've already made back your membership fee right there.

But if you're not careful, you could end up losing money at Costco instead of saving money. Here are a few ways you may be setting yourself up for that trap.

1. You're buying the wrong things in bulk

You could potentially save a lot of money by purchasing groceries in bulk at Costco, but that's only if you buy the right groceries.

If your household goes through eggs, fruit, and cheese quickly, it probably pays to buy those items in bulk. But it doesn't make sense to buy items in bulk that you only eat on occasion. And if you buy the wrong things, you risk them spoiling before they're used up.

Imagine your family eats tofu once a month, and a single container at your local grocery store costs $4. At Costco, you might score a four-pack of tofu for $10, bringing your cost per pack to just $2.50. But if you only end up eating two of your four packs before they spoil, you'll end up wasting $2.

The reason? It would only cost $8 to buy the amount of tofu you need at the supermarket. Instead, you're spending $10 on Costco's four-pack.

This is just one example. The point, however, is that if you're not careful about what you buy in bulk, you risk throwing money away. As a general rule, it's best to only buy things in bulk if you eat them regularly.

2. You're giving in to impulse purchases

Have you ever walked into Costco for $60 worth of groceries and come out with a $200 credit card tab? There's a reason that happens.

Costco does a great job of varying its inventory and offering up great prices. But that tends to tempt customers into spending more money than planned.

During the company's most recent earnings call, Ron Vachris, President and CEO of Costco, said, "We never want to compromise the treasure hunt of Costco." He also said, "You've heard the phrase, people come in to spend $100 and walk out with $300. That's because our buyers and our operators do a great job in making the warehouses exciting."

Giving in to impulse buys could be costing you more money than you bargained for. A smart move is to plan out your meals and purchases ahead of time, make a shopping list, and only set foot in aisles that are required to fulfill your list. Getting organized with meals could also make it so you're less likely to buy the wrong food items in bulk.

3. You're shopping with the wrong membership

While you can pay $65 per year for a basic Costco membership, an Executive membership costs double at $130. But the Executive membership gives you a big perk: 2% cash back on your Costco purchases. And if you're a pretty regular Costco shopper, you could be losing money by virtue of having the wrong membership.

Say you go to Costco 40 weeks out of the year and spend $100 per visit on groceries and household products (which isn't inconceivable given that you're buying a lot of bulk items). That's $4,000 a year in spending for run-of-the-mill purchases.

With a basic membership, none of that $4,000 comes back to you. With an Executive membership, you get $80 back. Even when you subtract the $65 it costs to upgrade to the Executive membership, you're still ahead by $15. And while that's hardly a life-changing sum, why would you want to give up any amount of free money?

There's the potential to save big by shopping at Costco. But make sure you're not accidentally losing money by falling victim to these pitfalls.

Top credit card to use at Costco (and everywhere else!)

We love versatile credit cards that offer huge rewards everywhere, including Costco! This card is a standout among America's favorite credit cards because it offers perhaps the easiest $200 cash bonus you could ever earn and an unlimited 2% cash rewards on purchases, even when you shop at Costco.

Add on the competitive 0% interest period and it's no wonder we awarded this card Best No Annual Fee Credit Card.

Click here to read our full review for free and apply before the $200 welcome bonus offer ends!

We're firm believers in the Golden Rule, which is why editorial opinions are ours alone and have not been previously reviewed, approved, or endorsed by included advertisers. The Ascent does not cover all offers on the market. Editorial content from The Ascent is separate from The Motley Fool editorial content and is created by a different analyst team.Maurie Backman has no position in any of the stocks mentioned. The Motley Fool has positions in and recommends Costco Wholesale. The Motley Fool has a disclosure policy.

3 Ways You're Losing Money at Costco Without Even Realizing It was originally published by The Motley Fool