I've never heard of either one, but...30 cents per snail? Considering your intended purpose for them, that's about 29.9 cents per snail more than I would be willing to pay for them. Personally, I would have gotten the snail-breeding going before actually getting the puffers, but whatever...snails are available usually for free for the common "pest" species, and they are considered pests because they breed like mad. It's still going to require a significant tank volume devoted to this project to be able to produce enough snails to use them as much more than just an occasional treat for multiple puffers.
Edited to add: when you say "posthorns" are you referring to "ramshorns"? And are "tower shells" by any chance Malaysian Trumpet snails?