Not a lot, you should have explained that to buyer before agreeing to post. He would have received a code and that is the only way you can prove delivery.
The recommended way for resolving this sort of situation, is with buyer’s agreement to cancel sale which refunds, then relist with postage as required and buyeri buys again.
Long winded but no other way to prove delivery, but it is time ebay came up with another option as this does happen a lot.
