Contact Customer Service explain what you have received that it's not useable, and hopefully they can reissue a return label and charge it to the seller's account.
I strongly recommend only contacting CS first thing when lines open for ' live agents' on a weekday at 8 a.m. as there's more chance of Dublin answering at that time, who will be more familiar with postage options in the UK, than the Philippine call centre.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
As said, I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@bow_131557