[iluvfish2]

I'd try and contact that Post Office and see if they cant reclaim it) - Its possible the PO Box owner will see its not theirs and put back in the mail for redelivery to the right address (so put a tracking tag on that package through their website). So far my experience with USPS will ultimately tell you they screwed up, but because its 1st class mail not much they can or will do. If its priority, then they just tell you to file a claim.

This is one aspect where Ebay will actually pay the refund and not typically force the seller to do it, as they've done what they were supposed to do, but its the 3rd party that screwed it up. But you may want the seller to check with eBay and see how they recommend to proceed.