I'd check the weather and look for a window between 60-80, preferably the cooler part of that range. Hot is more dangerous than cool. Double, triple bag the snail and water. A leaking bag is no good. Pack everything in snugly, bubble wrap or news paper are good to fill open spots. Opt for 2 day priority. I kind of hate to advise this, but I'm getting to the point where I no longer label "live fish," etc. on boxes. I don't want to think that there are cretins working in the postal facilities that would knowingly harm a package because there was something living in it, but I've had way worse luck with labeled boxes in the last couple of years, both sending and receiving. Could absolutely be coincidence, but I want my shipments to disappear into a sea of similar boxes and draw as little attention as possible. When the postal worker asks me if anything is "liquid, fragile, perishable or hazardous," I say liquid and perishable, and the boxes are stamped as such. If they ask, I say it's an aquatic plant. Hope your shipping goes smoothly!
Nothing good happens fast in an ecosystem.