I just rebooked a June rental using Costco codes as the price went down a few dollars for us. For the first time for us using the codes the form requires entering a Costco member number before the reservation was accepted. Not a problem for us as we are Costco members. I've never been asked for this in the past, and never been asked for a membership card when picking up the car. Just be aware that they (Alamo) are now asking for membership verification, possibly even asking for proof when picking the car up.