With coupons, I wouldn't worry about it. I would do a partial refund if the customer asked for it in this instance, but if they don't send you a message saying, "I forgot to use the coupon code! Is there anything you can do?" then I wouldn't worry about it. You don't get an automatic refund if you forget to use a coupon at the fabric store, and they always have coupons available.
I wouldn't worry about it.