Most marine snails in the pet trade won't survive at that salinity. Similarly, the Pomacea frreshwater species that are commonly sold as apple snails won't be happy in brackish. However, some Nerites are quite happy in brackish conditions. A lot of supposedly "freshwater" Nerites actually do better in brackish environments. Neritina reclivata (olive Nerite) and Neritina natalensis (tiger or zebra Nerite) are two such cases. If you get them in full freshwater at the shop though, which is the norm, you will need to acclimate them over a period of a couple of weeks to be safe - don't just toss them straight in. However, there is an additional complication, and that is the crabs. I don't know specifically which species you have, but some hermits and true crabs think snails are a delicious snack. Do you know what species of crabs you have or can you post pictures of them?
EDIT: the reason I ask for pictures is that common names for hermits and true crabs are very ambiguous and often misapplied at shops.