Originally Posted by AutoSlash

You may be asked to show proof of membership at the counter, but it's often loosely enforced. The problem is that if they do ask for it and you can't prove you are a member, you can be charged the walk-up rate which is far more than you can book for on-line beforehand. Often there are other discount codes that are available for public use that can get as lower or lower than the Costco code.