If you receive orchids from Hawaii
In the past, orchids from Hawaiian growers have had the small Bush snails in the medium and on top of it. From info out there at the time, they may be everywhere in Hawaii, and there's no way commercial growers can get rid of them.
Regardless, people repotted their plants before putting them in with their other plants (and checked them/sprayed them a certain number of days later?). When repotting, they looked closely for the small snails and washed them off.
Some sprays claim to kill snails. Apologies - exact remedies are escaping me, in case you don't get all the critters the first time. I had some but they didn't last long.