It's pretty common to get the small ramshorns or bladder snails along with other things from the LFS, because they hitchhike on plants and even on the shells of other snails. Ramshorns don't reproduce as quickly as bladder snails do, and personally I like them because they do keep the glass clean, don't eat plants and don't make a great deal of mess. Mystery snails, among my favourite snails, are amusing and attractive, but they are also rather messy, producing a fair bit of waste. Sort of like goldfish though not quite as bad as goldfish are.