Can a massage therapist charge a client interest for unpaid massage therapy treatments?

Sure. It is a standard business practice to charge interest on an outstanding balance. This is very similar to the interest credit card companies charge you when you only pay part of your balance. Though most massage therapists are compassionate and understand that their clients can have financial difficulties. Even though they have the right to charge interest, it would be more compassionate not to do so. People that get massages are already under a quite a bit of stress from their daily interactions, it would be nice to have a haven of healing and compassion where they did not have to worry about interest on unpaid bills. I personally would not charge interest.