LOL Well, by now you've probably done a Google search and figured out that the "funny snail" is a marketing name rather than anything meaningful. It may be that the LFS just doesn't know what to call them. If they are "Big Sand Sifters," that probably limits the possibilities. The most likely candidates are the large Nassarius species (most common is Nassarius arcularius) and a couple of Babylonia species. Both of these are generally safe, although both may be predatory under some circumstances. I wouldn't put them in the tank with small tridacnids, and I wouldn't put the Babylonia in a tank with any snails that were rare and special and that I didn't want to risk. They might also be another harmless sand dweller like Cerithium nodulosum, or they might be some other predatory snail that the LFS can't identify. If you could get a pic we could probably help you out with some more specific and dependable info.
Cheers, and GL,
Don