There are other external applications that can automate this.They can pull invoices and automate the sending of mails based on certain triggers and dates (for eg x days before due date ) etc.i have written about this .Check here - <a rel="nofollow" target="_blank" href="http://www.payorcrm.com/automated-payment-reminder-work-flows-and-quickbooks-credit-customers/?utm_s...>