Email Alert for due date reminder..

After the sales order is invoiced for a customer, Email alert should be triggered to the customer based on due date i.e. 15 days and 7 days (if not paid) before the due date. If the outstanding of the customer is based on different project, the respective sales account manager should also receive the email alert. Can you help me on this with clear step by step process to approach the mentioned. Thanks in Advance…