WOW, that parent has an attitude that does NOT sit well with me!! I'd want to tell her off like you did but I agree with the other ladies that it would be unprofessional. I would have just left it alone and said thanks for the interest but I'm not the right fit for your family. Our business relies on word of mouth the most so we need to keep that in mind when having discussions with potential clients and not let our emotions get the best of us. She has the right to think your late fee is high but she doesn't have the right to make you feel bad about it. It's your business so you can implement whatever rules you like (within reason and without scaring of the majority of clients of course!)

I think both of you were rude to each other without need to be but I'd shake it off and move on.