It is their business and they are entitled to run it how they want, even if it is not ideal. I agree, testing the animal prior to shipping would save a lot of time, frustration and heartache. However, if what they say is true about different strains of the virus, including those that have not been discovered yet, the testing results would only limited to what cam be found. Meaning, in their perspective, they may still be shipping out a potential sick animal after it was tested negative for a strain that we are aware of and limited to the test capability. Again, it is their company. Maybe they will look into this after their conversation with you. Maybe not.
I find changes are more likely to happen if it hurts the business bottom line. Even if you did not want the refund, some sort of compensation on their end shows that they are taking you seriously. Because they did not offer, and you did not push for it (You could donate the money to a reptile rescue), unless they step up and take some responsibility, there is no reason for them to change just because you want them to. They make enough money and have enough loyal fans to keep doing what they are doing.
Sucks for the poor snakes. I am sorry that this is happening to you and to these animals.