Concerning the diagnosis, internal ulcers are not the same as external and the type specified by the necropsy report were in the intestinal tract (large and small intestines), which has numerous bacteria (like salmonella) and digestive enzymes which contribute to ulceration in larger amounts than normal. Most, if not all reptiles, have an increased amount of these digestive enzymes to digest whole prey items, carion, and detritis that all carry an increased number of bacteria (think of it as increased stomach acid to kill the bacteria and make it safe for the animal to eat). So the point that the ulcers were caused at day 3, or before shipping is moot. We don't know, the vet doesn't know, so it's open to speculation. The only exception to this would be IF the vet had told Xavier that the ulcers were EXTREMELY large. That would be the closest to proof that they were preexisting.
Just because an animal gets sick once, does NOT mean it has a higher potential to get sick again. In fact, sometimes it's the exact opposite, especially if it develops antibodies towards a certain virus. I can't speak for anyone else's collection, but in my experiences as a vet tech, and in my own collection, this has not been the case. I have seen one or two cases where there's an underlying immunity problem where the animal is predisposed to a certain condition, such as RI and is more succeptible if the temp drops briefly. But I have treated numerous animals for infections, viruses, and parasites that have never recurred and some that may recur one or two times. It just depends on the animal, the environment, husbandry, genetics, age, etc.
I also wanted to add that each breeder has his/her own personal live guaranantee policies. Mine is 7 days and its stated on my website. So in this case, it wouldn't be covered. However, I would still try to meet them halfway because the situation is a gray area- the snake being so young, being bred before shipping (stress) and the other half- being shipped, flown, bred, and in an unfamiliar environment with no adjustment period (of even a week) (more stress). I'd have to take a percentage of that responsibility. I think this is just one of those situations where you have to look at the policies and the situation carefully and make a decision based on what you think is right. When you have money wrapped up in a live animal, that's the risk involved (on both sides).