I believe the snails sold as "mystery snails" are apple snails -- if so, here's a great resource that will tell you all about them, including breeding. http/www.applesnail.net/content/care.php#breeding
I've successfully hatched apple snails on a few occasions. The snail laid eggs on the underside of the aquarium hood and I didn't take them off fast enough, and they hatched. I had what seemed like *hundreds* of tiny shells (just smaller than a pencil-eraser) in my tank. They take a pretty long while to grow to a significant size too, so you'll want to keep that in mind