I have a client that maintains that I can't charge finance charges (I'm still researching where he got his law degree).  Of course, he is a real prompt payer ---------------------- takes about a year for him to pay.  I know he is ready to come in with his current year return when I see the payment arrive for the prior year.  He would always ignore the finance charges when he finally did pay.  A couple of years ago I wasn't feeling generous and when I billed him for the current year return I added a year's worth of finance charges into the current year bill.  Problem solved.