You'll need to read up about Simple Delivery which is now mandatory for all Private sellers, and the link below will explain how eBay handle all the postage / prices and allows currently, 2 postal services Royal Mail and Evri to be used. Some items are exempt and Custom Postage can be used, but I won't lie, there are presently major issues there! 🙄
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?mkevt=1&mkpid=2&emsid=0&mkcid=8&bu=43952...
Also worthwhile to go to eBay News top left of this forum page, and to Announcements read the latest on Simple Delivery, as there have been changes and updates to SD policy.
@pichonman2014