Anything automated for commercial interest accrual is a difficult one to solve and I have doubts on how effective it would be if we were simply auto-accruing interest as an additional line on an invoice. The interest is usually calculated daily, but how often would we update the interest value? Every update should require the invoice to be reissued again to ensure that the client is aware of the new balance to pay.
One possible solution here is to provide an option for overdue invoices to show a running interest figure (but only on the admin side), at any point the admin could convert that interest figure to a new invoice and QF would automatically handle the invoice creation and send. At the moment this is the only sensible option I can think of right now without things getting hugely complex. I’m open to any other suggestions here too!