Hmmmm, very tough situation you're in....
Unfortunately, I feel both you and the seller share responsibility.
As a buyer, you should know some basic health info on the animals you are purchasing and know if it's too warm or cold to be safely shipped. Mostly, if their policy was there before you made your purchase and you didn't read it, you can't really blame the seller for having a crappy policy like that. It's up to you to read the fine print.
On the other hand, the seller should know better than to ship animals in inproper and unsafe temperatures. I feel it's much more unforgivable from the seller's point of view to not care so much for the well-being of the animals they sell during shipping. It does not speak well of their practices if making room is more important than properly shipping animals. They should have pointed it out to you beforehand...
I have not seen such policies before, almost everyone guarantees live delivery with proper shipping methods.
This may be quite hard to fight in court because of their policy. It was there and you didn't read it. The judge will most likely fault you in the case based solely on that fact alone, that's why companies write out policies, to avoid problems after something goes wrong.
The only thing I can see you fighting the case on, is the seller as a provider of animals should know how to ship their animals safely. If they knew it was a risk to ship in extreme temperatures, it may be considered some form of abuse to have done so anyways.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!