I'm so sorry to hear this. It sucks being disappointed I know, especially when you lost something so valuable.
I recently lost a snake due to heat also. It was a complete accident but a long story.

The best advice I have is this;
If the snakes don't survive, and you would like your money back, then keep the receipt (if you have one) of the transaction between you and the breeder. Take the receipt to FedEx and see if they will resolve the issue. FedEx typically has good customer support, I'm really sorry and shocked that you had to go through this. Hope all goes well, please keep us updated on everything and let us know how everything goes! I will be thinking about you today.