Jim

You may explore https://apps.odoo.com/apps/modules/11.0/popup_reminder/

Hope it will help you.

Thank You

hi brother ,  

I would like to implement automatic reminders for customers who haven't paid their invoices at the due date.

if you have the project can you send me please

 thank you

I would also like to debit the expenses incurred for reminders due to late payments at the second reminder.

Thank you

Look at the module account_followup, it lets you define different levels of severity (how late a customer is) and specify what kind of action to take (email, call, manual..). It also creates and extends some useful views to see customers with unpaid invoices.