There are three optional reminders for an outstanding invoice.
Head to the Template Settings to edit them.
You can adjust the content of the message, how many days after the invoice date will trigger the message to be sent, as well as if you wanted to be copied in to any emails sent.
For the emails to be sent automatically, simply set the dropdown to "Yes".
Now any future invoices you produce using the software will automatically send out reminders to the customer.