As a matter of fact, I BELIEVE that by law you as a debtor (not a credit card or bank) are not allowed to charge a rate more than something like .9% per month for late fees on your invoices without having a preagreement with the customer to allow more. Meaning you MAY get more but without an agreement prior if the individual wants to fight it, you lose all but the .9%. It's been a while since I learned this so I'm a little sketchy on the details. All I really remember is that I tried to do the 18% myself and ran into some trouble with it.
This would most likely be state law.
And I'd get out of it due to the 'preagreement'. I'd have a written contract.
It sounds like the .9% would be if I, as a business, offer to bill people. IE I do some electrical work for somebody and bill them $400. They don't pay up in 30 days, I'm allowed to charge .9% per month. That's ~10% interest per year, depending on compounding. Assuming I don't do something different in the contract they sign before I do the work.