It must be hell for them though because you know what postmen are like, theyre not gentle with the parcel , even if they say, "FRAGILE". And also sometimes, things get lost in the post so if by any chance they get lost they'll starve in darkness in a little cramped box.
Posting them isn't ideal but the result is a snail that lives in a dedicated home with a huge variety of food, all the nutrition they could want with good conditions, free from predators. You could aliken it to refugees travelling in cramped conditions to escape to a safer country and hopefully a better life.
Snails are generally nocturnal and can aestivate/hibermate for a very long time. Being buried makes them feel safe from bad conditions and predators so I don't think that's a problem. Being upside down would be terrible for normal livestock but they frequently move upside down and they could correct themselves anyway. Most of the journey will be stationary - on a train, plane or van etc. Only a tiny percent of the journey will be the bit where they may get tumbled around. If you read articles on wild snails they often drop from trees if disturbed by predators so that's not completely without precedence in the wild. They obviously get over mild trauma like that.
Like I said, it's not ideal but done correctly, I don't think it's that bad at all...