It's been my experience when using a 12F coupon or Free Rental Days, that if my bill is over $1, then I get the Rental Credit Reward. The only time I have not received it is when I rented, using a 12F, from LGA and the total cost for the rental was about .45 cents. I did not receive any Rental Credit Rewards for this rental.
I've had 5 rentals (of varying length) this year using 12F coupons and have received a Rental Credit Reward of 1 day on each - the most recent being a 1 way from OXC to JFK
I've had multiple rentals this year (of varying length) using Free Rental Days, and have always received the Rental Credit Reward of 1 day - the most recent being a 5 day rental of a Camaro Conv at PHX