Hi Super,
Because they failed to accept the package, it's their responsibility to pay to reship. I somehow doubt they are going to want you to. If they haven't responded to your emails to pick it up, they probably aren't going to respond to pay you more money for the same package.
Fiest, I would pull their contact info and call them. ADVANCED SEARCH (upper right hand corner)
--(left side) FIND CONTACT INFORMATION
-enter ID. You may also need the item number. Ebay will send you an email with their telephone number and they will in turn get yours.
If that doesn't work, pkg comes back to you, and you've still had no contact from them, You could refund them, MINUS your shipping charge. After all you have tracking that proves you DID mail it and it WAS delivered, and emails reminding them to pick it up...not your fault. Good luck!