Wow, I ran into the exact same situation! The Petsmart near me is fantastic, the main aquarium area employees I've talked to know all about snails, ghost shrimp, bettas, etc because they own them and seem concerned about them. They actually have a beautiful clean mystery snail tank along with all the rest of their fish tanks, which honestly they have the best fish area of any Petsmart I've seen.
I decided to pick up a second mystery snail, but happened to be in another area, so went to a different Petmart thinking they'd all be good. I couldn't find the mystery snails, so I asked an employee. He said they were all in back and he could get me one. The one he got me looked about dead and had a large crack in back. I stared at it not sure what to say- this was really the best snail they had? I don't like confronting people, so I took it and figured I'd just talk to "my" Petsmart people about it. During the time I had him (in a betta cup), he popped out and seemed quite active, so at least he wasn't ailing.
I brought the snail into them and they said its likely he could heal, but I could exchange it if I wanted. I decided no one else would pick a cracked snail plus he was starting to grow on me, so I kept him and he ended up healing up great

I now make a point to try to spot where they keep the mystery snails if I'm in a store I havn't been in before. Another store had them in a small insect/hermit crab (?) cage inside the plant tank. There were a lot of snails squeezed into that small container, which was very old-looking.
I guess I'm lucky to have such a nice store near me.