I've returned plenty of unopened food items to Costco. As long as they're within the sales window, it's been no problem. So, like, my husband would bring home a new iced tea mix and say he was sure I'd love it. I'd read the ingredients, see Stevia, and say, "Nope. It goes back." Or, we'd bought Kirkland maple syrup, and my dad surprised me with fresh-from-the-sugar-shack Vermont maple syrup. I mean, the Kirkland is good, but... it's not Vermont maple syrup. I think the only things I took back past their sales window were things that either spoiled too fast -- a bag of fruit that went moldy in 4 days -- or, in one memorable case, that DIDN'T. We bought bananas that were still green, and they never turned yellow. The customer service rep gave us the helpful tip of checking to be sure that there is at least some yellow on the bunch, or they may never change color.

In general, I only return things they can still sell, or things that were clearly their fault. If it was my fault (for example, the fruit that was accidentally left in the car overnight and SURPRISE! rotted), I won't take it back.