If you want to send payment reminders automatically by e-mail, you can simply set up the Automatic Mail app. In principle, you can send as many automatic e-mails at different times.
Go to Apps and install the app Automatic Mail. You can then select which e-mail template should be sent at which time.
Here, for example, you will see the e-mail template for payment reminders 18 days before the start of the booking. The dispatch time can also be selected, for example, once per day or once per hour. To reduce the system load, a larger delivery interval is better.