DO NOT.
DO NOT.
DO NOT.
DO NOT.
Sand dollars will die in captivity for the same reason so many sea stars die: they are sand sifters and there is NOT enough food in a typical reef tank to keep them.
Also: you will NEVER SEE THEM. They stay buried in the sand. You'll spend money on them, they will immediately burrow, and be gone from your sight the few short weeks, or month or two they live.