When you get the package back, you reimburse the buyer.
EBay does allow you to charge a restocking fee, and in my opinion that should equal the cost of shipping. Your buyer might disagree and leave appropriate feedback.
Was there anything leading up to the payment that indicated that the buyer was no longer interested ? For example, did the buyer ask for a bid cancellation before he won? Such requests should be agreed to immediately, to avoid exactly the situation you are in now.