Here below are some of the main features for the configuration of the follow-up levels in Odoo V13:

- The template of the email is not made to be customized unless you add the fields from the tab "How to customize your messages"
- The invoices have to be sent or printed for the follow up levels to work.
- The emails that are sent out are grouped by partner, Odoo will base itself on the invoice with the oldest due date and gather the rest of the invoices on the same email. Once the invoice with the oldest due date gets paid, Odoo will base itself on the invoice with the second oldest due date.
- For the second reminder, third reminder and so on, Odoo will base itself also on the oldest invoice until this one gets paid and after, it will base again on the oldest due date to retrieve the mails.
- The automatic email on follow up levels does not work retrospectively, that means, if you have an invoice that has already reached the due date and after you create the follow up level, the email will not be sent out but you will be able to send it manually from Customers > Follow up reports.
- On the other hand, if you have the follow up set to : "After -1 days overdue, do the following actions:" and the due date is the 3rd of December, being today the 1st, on the 2nd of December, the email will be sent out automatically.