I own a business with 3 offices, in every office i have a Brother MFC 8680, i had to exchange one of those after 8 month of use, at the returns counter they called an electronics specialist to check. When he saw the box he said its a great device but if its acting up don't waste your time calling Brother cause their service sucks, just bring it here and get a new one. So following Costco signs and store policy is abusing it - i don't think so. If i decide i am not happy with a product i will go and exchange it for as long as i can, i am paying an annual fee for that luxury. If Costco decide they reduce the return policy on phones to 30 days or 14 days i might decide to shop elsewhere for my phones.
When they price a product they factor in returns and everything else - its called CDB - Cost of Doing Business.