Did the customer use a coupon code in the note to seller or bring up the coupon at all? If not, I would do a happy dance that they loved the item enough to pay full price for it and bought from you without even the incentive of a coupon! If they come back and say, "oops! I forgot to enter the coupon code!" I would just refund it via paypal. Or you could do that anyway, with a little note thanking them for their first purchase, and that first time customers receive a discount with their first purchase. It would be a fun (and memorable) surprise to them. Congratulations on the sale :-)