Please tell me you haven't refunded the buyer Before they sent the item back? If so, there is no way to enforce the buyer to now send your item.
For refunds the correct course, is for the buyer to open the appropriate return case. You then wait for the return of the item.
If it was for a case of item not as described you pay for that return postage. If for the reason doesn't fit / change of mind buyer here pays for the return postage.
Then, only on receipt of your item you fully refund through those case details and not other way. You have 48 hours to do so.
@pooky1916