Who refunded the buyer?
Did Ebay refund them because you sent the books with a Simple Delivery label or did the money come from your account?
The sets of books you sold early in November were listed with Simple Delivery.
When the buyer opened a case because they had not received the items, did you get a message saying the buyer had opened a case but Ebay were dealing with it and you didn't need to do anything?
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)