There are beautiful gift vouchers for sale at One Stop Nail Shops --- very classy looking and you can fill them in so that the client can purchase either services or retail items. They are very flexible.
Someone once came in to my salon with a gift voucher that was 2 years old!!! Needless to say, I made it a priority in the future that there was a time limit of 6 months only on the voucher. If it wasn't used by then it was tough cookies!!