As the seller he took the contract out with the Post Office, since they've returned it to him, it's his job to recoup any losses from them.
Put in a non-receipt of item claim through e~Bay and site that the seller is holding you to ransom over a 45 that you have already paid a large amount of shipping for.
You will get a full refund.
Then leave him negative feedback stating something like 'non~receipt' of item/unhelpful seller.