I send mine wrapped in filter floss with a bit of water all tied in a bag. If it's just MTS then I shove the bag in a jiffy and send that if they're apple snails or others with a thinner shell then I put the bag in an old takeaway tub with some newspaper as padding, then put that whole tub in a jiffy!
I don't send when it's absolutely freezing but I've put some in the post this morning now that it's slightly warmer.
My Dad had loads of MTS he wanted rid of, so took the gravel out and spread it out on the patio in the garden during the summer. Two weeks later, he rebuilt the tank, put the gravel in and two days after that, snails!