I suspect you're getting this at Costco because it's a better deal, in some ways, than getting it at Apple. That's Costco's business model, at least here in the states. Might be cheaper, might get more stuff with it, or it may come with a longer return policy. Or all of the above. At least here in the states, I think Apple's return policy is 14 days. Recently, my daughter's 2016 MBA failed, and she took it into Apple for AppleCare. It required depot. She's a student, and couldn't be without for 8 days. Their "loaner policy" was to buy a new one, and return It within 14 days.
I don't pretend to know how Apple or any other big company makes money, but this seemed a little crazy. But I know they know what they're doing.
I would take any product back to any retailer if it a) failed to meet expectations or b) came out with a better/newer/cheaper model within the allowable return period. But I get that there's a behavioral continuum on this - my wife prefers that I do the returns. If you're nice/respectful/courteous, never had a problem.