Payment due reminder is a setting for your reminders to set in the Invoice section.
You can change the setting as per your preference. For Example, How many days before you want the reminders to generate.
You can also ON the SMS and Email section for the reminders to appear. Also, if you want, you can set the email address reminders to another person by adding them in the Cc or Bcc field.
After the setting is done, click on Save button.