I've used both Alamo and National $20 --> $40 certificates numerous times at SJC.
It works GREAT!!!
I prefer National because even though their rates are a couple bucks more, I'm Emerald Exec, so I get to pay for the next lower class and YET use the certificate.
Also, if the rental ends up being > $0 (before taxes / fees), it counts as a paid rental and is counted as a credit to the National Emerald, rent 6 times, get 1 day free thing.
Enterprise has ALWAYS been a problem though.
I like Discover. hope you can get it to work for you!