It’s somehow the general belief that anything you buy online can be brought back to a corresponding bricks-and-mortar store, for a full refund, no questions asked. In some cases, it works. In many cases, the stores have a stringent return policy that prevents you from getting your money back, should you want to return an online purchase.
Costco has no such problems. If you bought an item in their online stores, you have all the right to take it back and get your money back as well.