Ive found theres two ways to do this though no doubt there are other ways.
The first is to get an empty plastic bottle, either one thats held painkillers or tablets, you know the clicky top things and that is perfect for a small number of snails. For a larger number which i had to send (20) i used empty vitamin tablet bottles. Rinse them out and then get some balls of cotton wool and soak them in tank water then place slightly wet into the container, just enough to stop the snails from rattling around and on top add the number of snails you send. I then put into a large brown envelope and voila there you go. A jiffy bag is a lot safer and at this time of year will keep them warmer longer.
Dont write on it what it is, just fragile would do and off you pop.
Another way is to put them wrapped in damp cotton wool straight into a jiffy bag and hope no one stands on it!!