I have a client who continually pays late and I have never assessed her a late fee, although it is stated in our letter of agreement. She was my first client and I don't want to lose her business, but she has yet to pay her latest invoice and I haven't heard from her all week. I always send her a reminder the day before her invoice is due.

My question is, how long should I wait before sending her a note to say I need payment in order to resume services? Her invoice is currently 3 days late. I do intend to assess the late fee this time, as she owes me quite a bit of money.