You will not get anything back you paid to promote your listing - those fees are not refundable and, as you have agreed to refund, you can't deduct the return postage.
When the buyer opens a 'not as described' case (however unfair it seems), the seller is liable for the postage.
"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)