Gift Certificates and Discount Coupons are two different things ...

Gift Certificates are for an Amount only ... and this is CASH that never expires until used by the customer ...

Discount Coupons can be for an Amount or a Percentage or Free Shipping ...

I think what you really need to do is make a Discount Coupon for 15% off and a 1 time use for the customer and send that out and not send out the Gift Certificate which would be an Amount for $15.00 rather than the Percentage you are trying to do ...