You told the seller you would ship it and you did, did you guarantee its arrival? Did you discuss how he wanted it shipped prior to sending it. If you have shipping receipt than I would say thats all you are on the hook for. You know you shipped it, you have receipt, he can file a claim with usps to obtain his item or refund. I would tell him you will work with him on his claim but thats it. He will leave negative feedback and then you dispute feedback with your version.