If you know how to what? refund credits? it is like this....if i win and i get it and it is bent i would file a dispute and say i dont want it and you click refund credits and then i mail your card back to you.
make the cardboard a bit longer and wider than the card. Place the card between the two slats, then use masking tape (or another stiff tape) to reinforce the cardboard. It might cost more than one stamp, but at least you won't have to refund credits!