I think it might help if you looked at this from another angle. Forget what other people have said or the principle of the thing for now, just focus on the deal being made between you and LLL.
Okay, so you said you agree with my post, and I am assuming you also agree with the reasons why sellers wouldn't offer to pay vet bills for customers - because some procedures customers get are unnecessary. Am I correct?
In my example, which was expanded upon by Nick, we explained that the radiograph was probably unnecessary to determine a case of mouth rot. You had it done just in case, for the good of the animal, but in the end it didn't really make a difference as far as determining treatment for mouth rot. If the snake had had broken ribs or something along those lines, it would have been a different situation.
In your email to LLL Reptile, you stated that you would be happy with the cost of the vet bill in store credit, plus some mite treatment:
If you subtract the radiograph from the vet bill, the cost of the visit and the medicine comes out to
$111.50. LLL Reptile is offering you
$199.99 in addition to mite treatment. The $200 (give or take a penny) covers the
necessary vet procedures AND most - if not all - of the shipping costs, which you didn't even ask for in this particular option. In addition to that, you ALSO get mite treatment, which you did request. They are essentially giving you one of the options you asked for, aside from paying for the radiograph. That seems like a very reasonable compromise to me, do you agree? I would accept that offer if I were in your shoes.
I have little reason to believe mouth rot and mites will kill this snake. The mouth rot wasn't advanced enough to be immediately apparent (like those poor snakes that can't even close their mouths), he's already being treated for it, and I have faith in you as a keeper to provide him with optimal conditions to heal in. You never said he looked rough in your emails to LLL Reptile, only that there was "something going on with his mouth", and you noticed a couple of mites. I understand it is unsettling to see
any health problems in your animals, but again, LLL Reptile should not be expected to cover the buyer's decision to run extra tests. That is the price of being a good keeper.

Their offer will compensate you for their mistakes - the mouth rot and the mites - so you are made whole as a customer.