As the title says the post office destroyed a card I sent in a trade.
The Trade: https://deckbox.org/trades/673234?s=75219
Pics of the card I sent: https://photos.app.goo.gl/j8c9Q9bVbr8Y1hg28
How do we go about finishing up this trade? Does it fall on me to replace the card?

The trading rules here say that it is the sender's job to make sure that the cards get to the recipient in good shape:

https://deckbox.org/help/trade_rules

But also note that if you use proper packaging (as specified here):

https://deckbox.org/help/how_to_send_cards

that you aren't responsible if the post office damages the cards along the way.

Did the whole toploader get bent?

In the past I have compensated people for cards damaged in transit as it seems like the right thing to do. If you were a store (or selling cards here), you would have to make amends for something damaged in transit. Extending that to trading seems proper, even if that's not what the rules for trading say.

If you sent it priority with tracking, it should have came with insurance. If so, you can file a claim with the post office to get some money back.

Did not send it with tracking. Just a PWE/Toploader with a team bag to seal in the freshness as you can see in the photos.
From the pictures it looks like the toploader was absolutely murdered while it was in transit.
According to the trade rules you linked I am in the clear but I do feel obligated to do something.

What do you think would be fair for me to do? I'm thinking 50% of the trade value in paypal?