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:
You can turn off SMS and Email Reminders for individual customers by selecting 'Disable Invoice Reminders' in their customer settings:
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.