Invoice reminders allow you to send your customers an email reminder about their invoice. You will be able to remind them before or after it’s due, your choice.

In order to get started first step is to go to Settings & Configuration => Invoice

Check the trigger for Enable partial/Unpaid Invoice Reminder

 

In order to send reminder for unpaid or partially paid invoice
 Goto  => manage invoices,

Select the unpaid / partial paid invoice and press action button, under action button click on send reminder to proceed further 

 

When you will click on send reminder, invoice reminder templet will open, I will be sent to customer when you will press send.