I charge penalties for late payments. They are defined in my contracts so they really are not arguable. Some customers always pay late and never pay the penalties unless I get after them about it. Particularly other contractors. Sigh. If you have to be late fine. I understand, but pay the price of it.
Anyway, I got an annual monitoring payment from a contractor today. They paid the penalties. Including ones on invoice all the way back to 2006.