I am in Michigan and we did this at my last Employer. We charged a $5.oo late fee. The law here says you can charge the patient, but if they don't pay it you have to write it off. But it does encourage people to pay. It's like they say in their heads wow, they must be serious about me paying.
So if you send them to collections, you can't use that late fee as part of the balance.
We also charged $25 for no shows. You would be amazed at the people that actually paid those.