If the buyer started a return on ebay ask them if they can add the postal tracking number to it. If not ask them to contact ebay and they will do it for them. They can can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
You can send them the link.
If they didn't open a return they can do that now and then contact ebay to give them the tracking number the used so it can be added to the return.
