Presumably, as the seller has only refunded the item price and not the postage then it must have been sent using eBay's Simple Delivery. If that is the case then it is eBay that will need to refund you for the postage.
As you have already been refunded for the item I think you will need to contact eBay Customer Services to explain the situation. They will be able to look at the tracking to verify what has happened and hopefully action a refund.
You can request a call back using the following link and I would recommend doing so first thing in the morning so that you get someone from the team in Ireland who are more knowledgeable and have greater authority:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
8am to 10pm on weekdays and
9am to 6pm on weekends
If that fails then contact your payment provider (PayPal or Card Issuer).