I cannot use any coupon with my rental because I am renting for 15 days. There are minimum/maximum days that you can rent using a coupon. I know the maximum is 14 days but I'm not sure what the minimum is. I don't know how long you are renting for, but maybe that is why the coupon will not go through for you. All coupons seem to have the same restrictions. Good luck.
Shelly