It looks as if you sent the item using an Ebay Simple Delivery label which was paid for by the buyer.
If you used the label Ebay provided, then you did not need to refund the buyer.
If they had opened an 'item not received' case, Ebay would have dealt with the buyer and refunded them without involving you.
I don't really think there is anything you can do, unless you did not use the SD label and bought the postage yourself direct from Evri.
If that is what happened, you just need to keep on at Evri for your item back or compensation.
"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)