Discover (Discover Card) says that you can redeem $20 of your cashback bonus for a $40 certificate to be used to rent a car at Alamo, a $40 certificate to be used to rent a car at National, or a $40 certificate to rent a car at Enterprise.
However, when you make the reservation, you have to tell them that you are using it, and they can then say that they do not have any cars available, if they do not want to let you use the certificate. They do this even if they have cars available for customers without the certificates. If this happens, you lose the $20 that you used to get the certificate.
Enterprise's said they had cars. On Discover's site, I found the code (DISCBB) to tell Enterprise's website that I am using one of the certificates (
https://www.discovercard.com/cardmembersvcs/rewards/app/browsePartner?modeCode=CP092 ), and it says that "No black out dates apply" (
https://www.discovercard.com/cardmembersvcs/rewards/app/showAllPartnersTerms ). However, if I try to reserve online with the code, it says that there are no cars; if I then start another browser window and do not mention the certificate, it says that there are cars. So they do have cars, just not for anyone using the $20/$40 thing. I though of calling on the phone, confirming that a car was available, and then saying, "oh wait, I have this certificate", but the rules for the Enterprise certificate require you to use a special phone number and say that you cannot use it at the regular phone number. So they know as soon as your dial. Clever.
The only other rental car company for which Discover was selling the certificates was National. I did not check whether the certificates work there, because, at the airport where I am going, National charges over $40 more (without the certificates) than other companies charge there, so it would cost more there with a certificate than it would cost at another company without one.