Failed Royal Mail Collection

Hi, all.

I recently had a notification from Royal About a failed collection, after checking the parcel had been picked up. Not having the goods nor anyway of identifying where they were I immediately advised the buyers and gave him a refund. move on to the next day, for some reason i checked the tracking on the order and it now appears Royal Mail have the goods and processing the delivery, Is there a way to reverse the refund or is this just a write it off and learn from experience.

Unable to find any specific on Ebay that will actually help.