Posting one of each species may be especially useful if your mom has collected a good number or variety of species. Particularly if she had focus on species number or had collected any that were smaller than a dime (as micros tend to be rarely posted). Rather than post them as casual, I would recommend you post them from the specific beach you think she visited the most and use a date around the middle of the range like 1970, and then make a note in the ID (so that it can be clearly seen by IDers) that the specimen may have been found at a different but nearby location and at a different time.
If someone wants to mark them casual then that’s fine, it is technically correct. For the specific case of seashells, I wouldn’t post them as “casual” intentionally because time and location don’t need to be as accurate for them as it does for other organisms and IDers may be less likely to find your observations if you do.
Realistically, who knows how long that shell rolled on the beach before she picked it up? And yes, that might be precisely on the beach where you found a shell but that wasn’t where it was when it was alive. Many species don’t actually live in the surf but farther out to sea so it really could have come from anywhere.
Your local shellers might have different advice, but that is what I would do if I were you.