Hi,

You can create a cron job for this and check the whether any invoice has the due date is over, Then for the corresponding invoice, you can create an email notification for the customer.

For creating  a cron job/Scheduled action which will execute automatically you can check this blog: Automated action in Odoo

For sending and generating the E-mail from python see this: Finding and sending e-mail templates in Odoo

This will call a python function in the specified interval from the python function if the due date is over you can notify the customer via mail.

Also, you can check whether any modules available for the same in the Odoo Apps

Thanks