She probably won't take your call so leave a polite, business toned message stating her account is overdue and $$$ dollars are owed. If you don't receive the funds by xxxx date, then you will file your claim against her with small claims or you could send the amount owing to collections. I would also send her a registered letter to this effect. Yes, it will cost you money (which you can charge back to her/ or use as a deduction) but it will let her know that when push comes to shove Mrs. Nice guy turns into Mrs. Tough guy.