I am not a legal expert but I believe that the provider just needs to have notified the patient prior to the bill of what the interest rate is. For example, they must have a policy such as "a 2% interest fee will be added to any balances over 30 days" or something like that. The policy needs to be made available to the patient prior to them incurring the bill. The provider can't say after the fact that they are going to charge interest because the bill wasn't paid.
Again, I am not a legal expert so I'm hoping someone else will chime in!