I would have reminded her of your late fees when you sent your reminder. I would have also given her the new total with the late fees included when you got her response saying she got busy.

If we don't enforce our own policies no one else will. My families know I mean business come payday. I have no problem asking at drop off for my cheque, no problem charging my late fee although once they pay it once or twice it doesn't happen again because they realize they need to remember!