The Post Office can't supply postage labels for your buyers' returns.
Tell the buyer to open an Ebay return request. Then all you have to do is 'accept' the return, and Ebay will automatically send the buyer a tracked postage label (at your expense if this is for the return of an item that the buyer claims is not-as-described). The buyer then prints the label, attaches it to the package, and takes the package to the Post Office so that the tracking number can be scanned into Royal Mail's system, and into Ebay's returns system. When tracking is updated to 'delivered' by Royal Mail, that will update as such in the open returns request, and you refund through the returns request to end the process.
Everything you need to know about selling on Ebay, including how the returns system works, is in the Seller Centre (link at foot of any page). You also have the Help pages with their integral search to refer to (links at top and foot of any page).
@johnads1954
