4 Common Mistakes People Make at Costco That Cost Money

The average Costco shopper spends around $150 a trip. With that much money on the line, it's easy to see how Costco can be a personal finance boon -- or a disaster.

Shopping smartly, you can find some fantastic deals in those hallowed aisles. If you're making some of these common Costco mistakes, however, you may be losing out on savings. Here's what to avoid.

1. Buying things in bulk you don't use enough

One of the key ways Costco saves you money is by selling things in bulk. You're buying more, yes, but the per-unit price is typically much lower than you'd pay for smaller quantities at a big-box or grocery store.

The trouble here is that buying in bulk is only actually a good deal if you're using up all of the purchase before it spoils. Throwing out half a gallon of unused mayonnaise probably means you didn't really save any money by purchasing the large size, despite a lower per-unit price.

To avoid this problem, make sure you're only buying the bulk-sized items at Costco that your family actually uses. For perishables, these should be things you go through quickly and/or in large quantities. For non-perishable items, just be sure you're happy with the quality and have the proper storage space.

2. Not making use of the excellent return policy

Costco has one of the best return policies in retail. Minus a few exceptions (major appliances, computers, etc.), you can return nearly any Costco purchase, more or less at any time -- and for pretty much any reason.

What do I mean? Well, that package of cheese that turned prematurely moldy? Yep, Costco will take that back. The coffee maker you bought two years ago that suddenly stopped working? It'll take that back, too.

My point is that there's really no reason to lose money on something that simply isn't working out. If you bought it at Costco and aren't satisfied, return it!

A caveat for the return policy

Although Costco's return policy is quite generous, that generosity has its limits. Excessive returns or otherwise appearing to abuse the return policy can get you in some trouble. Costco could suspend or even revoke your membership.

There isn't a published limit on how often you can make returns, so you'll need to use a bit of common sense.

3. Assuming Costco always has the best price

I have definitely found some great deals at Costco. And there's no doubt in my mind that most folks could save a bundle switching from name brands to Kirkland Signature (often without sacrificing quality).

All that being said, Costco is never guaranteed to be the best price. And you should never be taking for granted that Costco's price is the best without doing your own comparison shopping.

While it's admittedly rare that I find big items more expensive at Costco, it does happen. Moreover, I can regularly find brand-name electronics and small appliances that are the same price at Costco as at other retailers -- ones that don't require a costly membership.

4. Using the wrong rewards credit card

As a rewards evangelist, this is something I feel like most people can improve on in general, but especially when you're dropping three figures a trip. Costco doesn't make it easy, though, since the retailer has some persnickety credit card policies.

• In stores: You can only use Visa credit cards.
• Online: You can only use Visa or Mastercard credit cards.

Costco's own credit card is a competitive option. Its gas rewards alone can be quite lucrative. However, it may not be the best way to maximize your rewards. A travel rewards card, for instance, may be a better bet. Some of these cards can earn more than $0.02 per point when the rewards are transferred and redeemed for free travel.

Consider your needs -- and other spending habits -- and choose a card that maximizes your rewards at Costco, as well as on other regular expenses.

Shop smarter, not harder

Costco's endless aisles of goodies can harbor a ton of great deals. But, as with many things, the deal is in the eye (or wallet?) of the proverbial beholder. Avoid buying things you won't use, keeping things you can't use, and using the wrong credit card. And always, always, always compare prices!