Hi Avomania
AccountRight has the ability to add a finance charge to a customer's statement using the Add Finance Charges to Amount Due in the Advanced Filters option in the Print/Email Statements window. This will add the value determined by the invoice terms and the %Monthly Charge for Late Payment - which you can see on the invoice by selecting the Terms arrow in the Edit Sales window.
Personally, I do find that method a little complex and it doesn't meet the customers' needs. For the most part, I've seen customers just generate another invoice for the customer and then keeping adding that finance charge amount to it next month or generating a new invoice each month for that.
