The biggest, costliest Costco mistakes you could be making By Lisa Maloney of Cheapism | Bulk-Buying Boo-Boos Costco is practically a mecca for bargain-minded shoppers. Sure, you have to purchase an annual membership to get in the door — but in return, you get access to a world of high-quality goods at great prices, all backed by a generous return policy. Whether you need a new household appliance, a set of tires for your car, seasonal clothing, fresh produce, or the biggest containers of Lysol wipes you've ever seen, you can find almost any type of product at Costco. But despite its l...