Breathing bags are what I use. They are fairly new to the hobby and some are reluctant to try them but I swear by them. They allow oxygen to go into the bag while carbon monoxide is expelled. I have had fish live as long as 7 days when they get lost in the mail. Some of my snails have taken 5 days and where still healthy on the other end.
I use the priority mail boxes that you can get free from the PO and add 1/2 inch insulation like is used in houses that I cut to fit. then I put newsprint in (can use packing peanuts as well) and pack it in there so the bags dont touch each other if they do it affects the oxygen/carbon monoxide breathing. then on the very top I wrap the heat pack in a single layer of newspaper and tape it to the lid. That way the heat pack dosnt get to close to the bags possibly causing overheating or melting.
Breather bags are reusable and they can heal small leaks on their own. Breathing bags work best with as little water as possible so that the fish are closer to the edges. You dont want any oxygen pockets in there either since the fish get sloshed more and can get caught in wrinkles. I actually tape the tips down on the bottom so that when the bag shifts there is no little spot to get caught in. Small fish can get trapped there and die.
With snails I put less than 1/4 cup of water in with a single snail then I take a rubber band and close that section then I take another snail more water and another rubber band. I can get 4 snails in a snail sausage in the medium bag and 2 in a small bag. I need to take pictures of my snausages to show lol
I have only lost 1 snail out of approx. 350 snails shipped this year. It was one I was giving away due to some minor shell damage but apparently it had more than I knew about

Everyone else has came out of the bag crawling with no shipping trauma that is seen when they are shipped without water.
I hope that is the answer you where looking for alex

ETA: Here are my snausages

I didnt put the snails in since I have touched snails enough today

but that is exactly what it looks like.
http://i251.photobucket.com/albums/gg283/boxer_isis/snausage/snausage2.jpg