QB has a simple but effective system to achieve this. You create an email template message and from the outstanding & overdue invoice screen, a PDF copy of the invoice is sent to the customer with your email at a pre-set numbers of days after due date. It can be limited to say 3 dates each month. This is automatic and needs no input from ourselves and means that we don't have to specifically look and monitor outstanding invoices. I tended to use 5, 12 & 18 days past due date but that would be down to each Sage user.