Was it the pump?
That sold with Ebay Simple Delivery so the buyer paid the postage fee directly to Ebay.
Because the label was not used, the buyer will be refunded by Ebay for the postage.
You can't access that payment as Ebay expect you to use the label they supply and the buyer pays for.
You could suggest to the buyer that they reimburse you for the postage when they get their refund from Ebay, but you can't make them pay you.
"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)