Do you want the ability to automatically send customers their Past Due Notices? 

Step 1 - Go to Business > Settings > Notifications

Step 2 -  Select 'Enable past due reminders' under the Past Due Reminders section to automatically send a Past Due notice by email to your customers;

Step 3 -  Select the number of days after the due date that you'd like to have the notice sent out.

Step 4 -  Select your customers to receive the notice by clicking   and then selecting them from the listing of available customers with an email address.