I had no problems using a $40 one for Alamo back in sept for an Orlando rental.
I got an amazing deal because I combined it with the $10 a day promo they
were running back then.
Just booked the car as normal and on payment screen clicked something like "have a coupon/certificate (I dont remember) " and it applied the $40 off.
Total was like $13 after certificate for a 4 day rental (off-airport at the sheraton) was a great, great deal.
Was not questioned at the rental place about it either. Charge on my CC was for $13 as promised.
Sounds like enterprise is more sneaky about it though. Have not tried 1 of those.