I use Dollar Express and the Renter Rewards program. You get one free day car rental for every 16 days that you rent. They mail you a physical certificate that you have to bring with you to the rental car kiosk when you pickup the keys.
The certificates have a one-year expiration date on them. I usually save them up and use them for vacation rentals (after earning them on work rentals).
I recently had one free day certificate expire. I called them and renewed it. They e-mailed me a 6-month extension letter that I have printed out & stapled to the original certificate. very nice customer service! I will enjoy the rental next month in Florida!