So, things are marked a bit different at the feed store I go to...
Is the "shell grit" oyster shell? If so, that would be what you would offer to laying hens for extra calcium.
I'm not sure what the benefit of the shell grit mix would be if it was mostly sand. Sand is not really large enough to help a grown bird grind their food.
If your birds are free ranging, and you are confident that they have access to pebbles, gravel and such to grind their food than you would not have to purchase grit (a good deal larger than sand) that is generally from granite to help grind their food. If your birds are confined to a run, giving them some grit in a dish, or just throwing it on the ground will be necessary.