If you are buying a slew of these (as I did) make sure you take the time to go through each envelope and make sure there are two $50 certificates in each envelope. The Costco supervisors are usually pretty good about doing this, but since you are going to leave with a bunch of envelopes, it is wise to take the time to methodically check them all. That $20% discount will disappear pretty quickly if you get shortchanged.
I discovered these on Friday night at the Dedham, MA location, and bought a stack - first time I saw the check-out register flashing "verify customer signature".