You can setup automatic invoice reminders to send either SMS, email, or both when an invoice becomes overdue.
Hop on over to ⚙ Settings > Reminders and scroll down to Invoices to turn on SMS, Email, or both:
Turn off SMS and Email Reminders for specific customers by selecting Disable Invoice Reminders in their customer settings (head to Customers on the navigation bar, select the customer you want to edit, and click the pencil button):
Click the SMS Settings or Email Settings buttons to edit the reminder templates.
While an invoice remains unpaid SMS reminders will be sent at the following frequency:
1 - The day after invoice approval
2 - On the due date
3 - 7 days after the due date
While an invoice remains unpaid, Email Reminders will be sent the day after the invoice is due then every 14 Days.