The postage doesn't go through your account if you use Simple Delivery - that is a transaction between Ebay and the buyer.
So, although I am not certain, I would guess that Ebay would refund the buyer the total amount they paid, including Buyer Protection Fee and the charge for SD.
I can understand why you are asking, but it is not something you are involved in.
"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)