Coupons only have an effect on the base price (accommodation fare) of the reservation. The percentage-off coupon type can be configured to discount up to 100% of the accommodation fare. All other fees will not be discounted. The only way a coupon can exceed the accommodation fare is if it is a money-off coupon type.
If the discount is greater than the base price, guests will see the following disclaimer before proceeding with booking the reservation with a money-off coupon: “The coupon value is higher than the booking fee. You won't be able to use the remaining balance.”