The jacket was listed with Free Delivery so you can request a refund for the postage here:

https://www.ebay.co.uk/claims/simple-delivery/refund/seller

As I stated in your other thread on the subject, which is still there, the Post Office shouldn't have refused the item as they are contractually obliged to accept all Simple Delivery parcels, regardless of whether the size/weight corresponds with the label, provided it is within the maximum weight/dimensions permitted by the carrier (as stated on the Post Office website).  It will still be accepted by the carrier as eBay has agreed a flat rate contract with them.

Ebay is introducing new size options for items that are large but not particularly heavy, which will supposedly be available sometime this week, so if you wait a bit you should be able to relist the jacket for a lot less postage than the current bulky item option.