Are you returning the item for the reason... item not as described? Then Do Not close the case.
You will receive a standard label for a 2 kg item, for a heavier item that you have, you need to contact eBay Customer Services, they will provide the correct label, and charge it to the seller's account.
To Contact Customer Services it's a Live Chat, or you can select to speak and request a Call Back. Click on this link:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
On the left sidebar you can see Have us Call you or Chat with Our Automated Assistant.
On weekdays lines open between 8 a.m. and 10 .m.
On weekends they open between 9 a.m. and 6 p.m.