Hmm, you have their address. You think you didn't refund the postage but you might have. How much was the postage? Why not just try the ole' U.S. postal system and mail them the refund snail mail? I often get payments via the USPS from customers I refunded due to the slower than molasses postal system only to arrive three weeks after I refunded them. People still write checks or throw a ten spot in the mail.
Or if you owe them a few dollars why not shoot them an e-mail stating that the extra postage can be used on their next purchase? Home Depot does that to me often in the form of a gift card.
