Sending payment reminder can be done in the invoice screen, simply click the "Check All" box at the upper right of the screen. This will mark all the unpaid invoices that you want to send to customers. If you need to exclude a customer, all you need to do is uncheck the box at the right of their name.
Scroll at the bottom of the screen and click the "Email Payment Reminders" button.
A new window will open that will allow you to customize the email you want to send through the "payment reminder settings". There is also a created template that you can readily use. When done editing, you can click the "Send Email" button at the bottom of the screen.
If you have any questions, please let me know. Take care and have a great day.