I sold an item which the buyer tells me is defected.  Without thinking about it I have offered to have the item back and offered a full refund via paypal.

The problem is, I quoted $15 for postage and it actually cost me $27.15 due to the size and location of the buyer, so I'm out of pocket, which I was happy to wear.  Now the buyer is going to post it back and I've refunded the orginal postage quoted of $15, so do I have to pay the full postage back or just the difference?