These 3 Costco Mistakes Will Cost You Money Instead of Saving You Money

Many people make the decision to join Costco for the savings involved. Although there's a financial outlay for a membership, it might more than pay for itself if you shop there throughout the year.

But certain Costco moves you might think are financially savvy could end up coming back to bite you. Here are three that might cost you big time.

1. Sticking with a basic membership

A basic membership at Costco costs $60 a year, while an Executive membership costs $120. You might assume that sticking to a basic membership is a smart move, since you're shelling out less money upfront. But if you do a lot of shopping at Costco, you might actually lose money by not upgrading your membership.

See, for the extra $60 a year your Executive membership costs, you get 2% cash back on your Costco purchases. So once you've spent more than $3,000 at Costco in a year, the Executive membership has paid for itself.

Let's say you shop at Costco most weeks and spend $100 each time you go. If you spend $5,000 during the year, you'll get $100 back on an Executive membership. When you subtract the cost of that upgraded membership, you're up $40.

To put it another way, in this example, you're spending $120 on an Executive membership but getting $100 of it back, so your total membership outlay is only $20. With a basic membership, you're spending $60 but getting $0 back, so your total outlay is, well, $60.

2. Buying items in bulk you won't get to use before they expire

Buying in bulk at Costco is a great way to save money -- if you purchase the right products. But if you buy items in bulk that you don't use up before they expire, then you're basically throwing money away.

A good rule of thumb when buying perishable foods in bulk is to ask yourself, "How many times a week does my family eat this product?" If the product in question is a case of strawberries and the answer is "five," then you're probably OK making that purchase. But if it's something most people in your family eat once a week and are then done with, it's probably not a good bulk buy.

3. Buying items online you can find in the store

Many of the items you'll find on Costco.com come with free shipping. You may be inclined to buy items on the site to avoid a trip to the store. But keep in mind that you actually are paying for shipping -- just not directly.

Most of the items that Costco sells both online and in stores are cheaper to purchase in stores. So when you buy a $15 item online with free shipping that sells for $11 at your local warehouse club, you're basically paying a $4 shipping fee without realizing it.

Now, there are some Costco items that may only be available online. If you have a need for those, by all means, place an order. But if a given item is available in stores, buying it online could mean wasting money.

There are plenty of opportunities to make smart financial decisions when shopping at Costco. But be aware that these moves in particular might leave you kicking yourself.