Join the club: 18 costly mistakes to avoid making at Sam's Club or Costco

Membership-based warehouse clubs Costco (NASDAQ: COST) and Walmart's (NYSE: WMT) Sam's Club can save members a lot of money. They can also cost you money in a variety of ways if you're not very careful.

If you join one these chains there are lots of ways you can end up wasting money. To avoid doing that it's important to be aware of some of the traps you can fall into.

Food can go to waste: Some foods have very long shelf lives. Others will spoil and some items -- like meat and fish -- that can be frozen, will only last so long even in the freezer. Since warehouse clubs sell in bulk it's important to only buy items in quantities you will actually use. If you end up throwing some of what you purchased away you didn't really save money.

Don't forget gas: Not every Costco and Sam's Club has a gas station. At those that do, you are likely to get some of the best, if not the best, prices in town. The location of your warehouse club may not be convenient for a regular fill-up, but if you're already there, it's worth filling up.

Watch for expiration dates: Both warehouse club chains offer significant deals on over-the-counter medicine and various supplements. Some of these, however, come in quantities that you are not likely to use before they expire. Check expiration dates and do some honest math as to whether you will use up that oversized bottle of aspirin or bucket of fish oil pills before they go bad.

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Don't get trapped: Sometimes warehouse clubs have really good prices on items you weren't planning on buying. That could mean you're getting a great deal or it could mean you're getting tricked into buying a life-sized teddy bear, stand-up paddleboard, or kayak you don't really need.

Try things on: Both warehouse club chains stock a rotating selection of clothing. What they don't offer is dressing rooms. Do the best you can to try on any items you may buy or only select merchandise you already own so you know your size. If you can't practically try something on in the store, do so as soon as you get home. If it does not fit, bring it back right away to get the right size before the club stops stocking that particular item.

Don't fall for samples: When you walk around a warehouse club there are often stations giving out free food. That one enjoyable bite can lead to you heading home with a sack of frozen meatballs you have no real plan to use or a big box of candy you almost certainly don't need. Enjoy the samples, but only buy the item if it was something you previously had planned to purchase.

Check prices: Warehouse clubs are usually cheaper. They do not always have the lowest prices on every item. That's not important if you pay a penny or two more for an item, but pennies can add up to dollars so make sure you use your phone to at least spot check prices.

Use it or lose it: A warehouse club membership does not save you money if you don't use it. Make a regular plan to visit your club and buy things that are sold at better prices than you can get otherwise. In many cases, bulk items like toilet paper or paper towels are a good deal where you don't have to worry about expiration.

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Shop with a list: Many people who make a list when they go to the regular grocery store don't when they visit a warehouse club. That's a mistake because it can lead to buying things you don't actually need. Make a list before you go. You don't have to stick to it rigidly if you see something else you actually need but give a lot of thought to any items that you did not plan to buy before entering the store.

Know how long perishables last: Just because you can buy something in a large quantity does not mean you should. Cereal, for example, can get stale quickly once opened and many condiments go bad after six months. Even spices lose efficacy after about a year. It's not a deal if you end up throwing half of what you bought away. In addition, even if you will use it, consider whether you want to devote the refrigerator space required by a tub of mayonnaise.

Be wary of electronics: Warehouse clubs sell electronics including computers and televisions. In some cases, their prices on these are not as good as what's offered by online or brick-and-mortar electronics dealers. In other cases, they sell a model that's similar to one that costs more elsewhere that has less features or other meaningful differences.

Consider quality: You can buy eyeglasses at Costco or Sam's Club at what are generally very good prices. That might be a good deal, but price is not the only factor you should consider when buying glasses. Will the pair you buy last as long as the ones you regularly buy? Is the quality of the frames equal or at least in the ballpark? Some pairs may not be up to par so take a careful look at what you're buying.

Beware the unknown: Warehouse clubs might offer brands you have never heard of (or house brands you have never tried) at very good prices. That can be a savings, but it can also lead you to end up with a lot of something you don't like that much. Both Costco and Sam's offer great deals on coffee pods or K-Cups. Sometimes these are familiar brands and flavors. In other cases, they are off-brands or house brands that may not taste as you expected. If you're not picky or are buying for a party or mass gathering, go ahead. In other cases, be careful.

Know the return policy: Costco and Sam's Club generally have generous return policies. There are exceptions, however, as both have a 90-day policies when it comes to most electronics. When you buy something out of the ordinary -- whether it be a kayak or a coffin -- know whether it's covered by the normal return policy or if it's an exception.

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Be wary of things you can't try: Warehouse clubs sell mattresses but in most cases they don't allow you to try them. In general, mattresses are displayed vertically, not in a way consumers can lie down on them. That's fine if you're outfitting a guest bedroom but it's probably not a great idea to take a risk if it's for your regular bed.

Don't skip over the cafeteria: Costco and Sam's Club both have cafeterias that sell cheap hot dogs, pizza, and other food. If you were planning to eat out anyway doing so at the warehouse club can save you some money (and many people rave about the food).

Consider the credit card: Both Costco and Sam's club offer rewards-based credit cards. The programs are a little different, but they offer cash back to customers for shopping at the warehouse clubs and in other areas including travel and gas. Don't take on a new credit card lightly. Look at what the rewards are and whether your spending habits will make the savings worth it for you.

Don't forget the websites: Warehouse club members have access to the websites offered by each chain. These sites offer deals not found in stores on everything from travel to insurance and business services. The two chains have different offerings and the merchandise offered changes regularly, so as a member it makes sense to get familiar with the website for your club.

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Daniel B. Kline has no position in any of the stocks mentioned. The Motley Fool recommends Costco Wholesale. The Motley Fool has a disclosure policy.

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