There is a lot of debate on shells. I would say no on the sand dollars (though I have no real knowledge, so that is purely a guess).
Shells need to be cleaned with white vinegar or bleach and warm water first. Some people use them, some don't. It has been known to alter the pH of the water. Bettas are pretty hardy when it comes to pH and as long as it isn't a HUGE fluctuation, it should be ok. Get a test kit (if you don't have one already) and just keep an eye on the levels if you decide to use shells. The general conclusion that people seem to come to are that if it's only a few shells, it's fine and keep an eye on it. If you use so many that it's practically like substrate, that's probably a bad idea.