Our costco requires a membership card no matter the form of paymen t- they are crackign down on those who are taking advantage of this and they ar eremoving the loopholes - we were just there on Sunday and someone was trying this at self-checkout andy actually referenced several "money saving" sites saying it was allowed and the Manager said they were aware of teh sties and Costro is removing that loophold and yuo will have to scan your membership card to continue the purhcase. Also, Costso requires the name on the card be on the ID of the person that is paying so you can't just borrow a friend's card and go - you can go WITH them as a guest, and give them $$ for your items though.
Our Sam's also requires that the name on the card (that comes up on their end) matches the ID of the person paying