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):
Settings
Click the SMS Settings or Email Settings buttons to edit the reminder templates.
Email and SMS Reminders
While an invoice remains unpaid SMS/Email reminders will be sent depending on the frequency you have set in the Reminders Settings.
Below are the options you can choose from:
On the day after the approval
On the due date
1 day overdue
5 days overdue
7 days overdue
10 days overdue
14 days overdue
21 days overdue
28 days overdue
42 days overdue