From some prior experiences:

If you rent for three days and return early but still have a Saturday night keep, the coupon still applies and you end up only paying for one day out of the two you had the car. (great deal when you only need the car 2 days over a weekend)

If you rent for three days and return early and do not have a Saturday night keep, the coupon comes off the reservation and you pay for all the time you had the car.