Was the return one for item not as described? Then, this should have been followed through with paying for their tracked return postage, refunding through those case details on receipt of the items. Then, relisting the items preferably as one listing, so one postage cost, for the buyer to buyer again.
If the above wasn't followed nothing else you can do now, and as long as the buyer doesn't escalate the case the case(s) will just time out and close.
@sandylilykay