When ever I have had snails delivered, they were in a small cardboard box with the snails being in a normal fish bag with some water (less than what you're suggesting), the box was packed out with news paper, and there was news paper over the bag too, the heat pack (approx.4x3")was stuck to the lid of the box.
With regards to the snails arriving alive, that's only a small part of the battle, it's the acclimation that'll make the difference.
as a side note, the nass snails are the only snail type that I have had 100% success rate with regards to acquisition and introduction into the tank. mine have laid more eggs! how often do yours lay eggs Mike?
those pots you're suggesting are great for stony corals BTW.