Ferryhopper discount coupons are valid for a single booking and can be used only once unless the coupon restrictions state otherwise. Coupons cannot be used cumulatively and are non-transferrable.
Each coupon has its own expiration date and cannot be used for cheaper ferry tickets once the designated date has passed.
Also, bear in mind that if you wish to change a booking completed with a Ferryhopper discount coupon, the coupon cannot be applied anew. This means you need to pay for any difference in price, including the original coupon value and any ticket price difference.