It is illegal to ship snails at all without a USDA permit. Depending on the species, you may or may not be able to obtain a permit. For Pomacea Bridgesii, permits are available for all 50 states still, and you can apply for one by going to the USDA website. It's free, I believe, and very easy to do.
For any other Pomacea species, they won't give you a permit, as they consider all other Pomaceas plant-eating pests. Spixies are very difficult to obtain permits for right now, and there's the possibility of a ban for them in the near future. Red ramshorns don't require permits, as far as i know, but giant columbian ramshorns do.