Automatic overdue fines cannot be set on overdue reminders themselves. Reminders with overdue fines can be created manually, however.
You can open your invoice (from which the reminder is to be generated) with the status "overdue", and press the button "Edit invoice again".
Now you can implement the item overdue fees directly on the invoice. Then press the button "Invoice is set" to put the bill back into the status "open" or "overdue". Manual Reminders can only be generated for invoices with the status "open" or "overdue".
Using the button "Change invoice to the next reminder level" under the tab "reminders", you can create a reminder with overdue fines.
Reminder already created, what to do?
If you have raised your Invoice already to the next warning level and want to add an overdue fee to this reminder, you can delete the reminder for the desired invoice under the tab "reminders", and create a new reminder with overdue fines.
It makes sense to handle overdue fines directly as a product, so that you can import this item with just a few clicks to the product position on an invoice.
For how to add a new product, see the report "Add new product".