Give them time - don't poke at them or pick them up to check them. They may perk up when the lights go out. If it's acclimation stress, there's nothing you can do about that except leave them be and wait. You can test water parameters but if you find anything off, don't try to fix it super fast or you risk stressing them even more.
How did you acclimate the snails? Drip acclimation is safest. With other methods, the larger the percentage change in water each time the more risky it is with snails. Marine snails can be very sensitive to snap changes in water parameters. Yours look to be Astreas which are on the more sensitive side from my experience. You also have to be careful how you remove snails from the acclimation container to avoid injuring the columellar muscle (which is what gets strained as they try to pull against you); injured snails will often sit mostly closed and refuse to move.
Was the water you got from the LFS a jug they filled for you out of a tap on the spot (indicated its water they mixed themselves for their own tanks) or was it something that came to the LFS sealed in its own single use container? The pre-packaged, single-use container ones can sometimes have a weirdly high salinity and sometimes needs to be diluted before use.