You can use Automations to notify your customers about past-due invoices.
To set this up, click Automations in the menu, then click the "New Automation" button.
Select "An invoice becomes past due" as the Trigger Condition.
Next, select either "Text Customer" or "Email Customer" as the Action:
Please note that you need to have Text Messaging enabled to be able to texts customers about past due-invoices. To enable Text Messaging, please visit the Text Messaging page by clicking "Text" from the menu.