You first must deal with your buyer and their refund.
Ask them to open a case for item not as described. You must accept this case within 3 days, and refund or if you want your item sending back first, you pay for that tracked return label, and once the item is back with you, you have 48 hours to refund, through those case details.
Stick within those timeframes, and your seller performance is not affected.
Then, as you bought your postage through eBay you make a claim with Packlink here:
https://support-ebay.packlink.com/hc/en-gb/articles/360014270619-EasyTicket
@alkho-60