posting snails

We're talking about Brigs/Canas?

If they're smallish (about the size of a 1p peice) I like to use the "foam sandwich" method. Get a smallish tupperware/gladware/rubbermade container and poke SMALL holes in the lid. Then get some quilt batting or filter foam, and cut a bit to cover the bottom of the container, then dampen it with dechlored water. Lay the snails on the foam/batting, then cover them with more dampened foam/batting cut to fit. Make sure you use enough to have to "squish" the upper level of the foam/batting a little when you put the lid of the container on. You can do more than one "level" of snails, if the container is tall enough.
I usually wrap the snails in wet paper towel, individually, before laying them on the bottom half of the "sandwich".
Tape the container shut, really well!

Place the container into a box that is lined with styrofoam cut to fit the inside of the box, and pack batting or shredded newspaper around the container really nice and tight, to prevent the container from shifting around. Cover the paper/container with another bit of styro and close the lid of the box. Tape shut.

Wrap in shipping paper, and send overnight, or 2-day express, whatever will get to the destination in a couple of days.

For larger snails, an inch or more across the shell, it's better to bag them individually in small fish bags, with a little water.