Question Author
That's a good idea Chris, but it's not possible. There's only two text boxes - one to insert a number and the other to insert the carrier's name. I tried typing in, "Buyer admits it was his error, he received the item" but it wouldn't allow it. It has to be numbers.
I messaged the buyer yesterday and he does sound as if he's willing to refund me, but being the suspicious person that I am, I wonder if he's genuine. eBay have told me that unless I respond with tracking details, he will automatically get a refund, so it looks like I've been stuffed.