To be fair, it might not have been Costco's fault, it could have been the last guys to put the wheel on who impact gunned the fuck out of it, or shitty studs or lugs, or a combination of all three. That said, Costco should have had zero problems replacing a broken stud on the spot, any business who regularly removes wheel from cars should be prepared to replace a few studs here and there. It would be like hiring a guy to change your light fixtures but he can't finish because he didn't bring any spare wire nuts because he assumes their are already good wirenuts on the old light fixtures.