Returned item where additional postage has been incurred

[acradavi]

I had reason to return a hoverboard because it was not working properly. The seller asked me to despatch the item to a UK address, track it for them and let them know when it had arrived. The addition postage amounted to £23.14 with Royal Mail. They have refunded me for the items full price but not the additional postage charge. The return has now been marked closed and the seller is not responding to my emails. How can I resolve this please? 

[temperate_porcine · Accepted solution]

You can't, a lot of sellers dislike paying return postage costs so they ask their buyers to pay, had you opened a case and escalated, you'd have received a postage label, there's no way to reclaim the postage now unless the seller does it voluntarily.