From the warnings others have given about this salt, I wouldn't use this type of salt at all. If you add it to the top, you need to tell people to cut off the top of the soap to remove the crystals before use. It has a good chance of lacerating the skin if left on or in the soap. Any kind of salt on top of the soap can absorb water from the air in humid weather. This will make the soap wet and goopy.
Really? That's weird. I use them all the time without any issues.