Unpaid invoice – how do I create a new invoice with reminder fees?

Hello,

I have a social services department that still has not paid an invoice with a due date of 9/5, even though I’ve already sent two reminders by email without charging additional reminder fees.I was promised that the invoice would be paid with the June payment round, but it is now 3 July and the amount has not yet been stated on my account. What happens now? I read on the forum here that you have to send another reminder by registered post. This time, I want to charge reminder fees. The amount from the original invoice was 375 euros. My terms and conditions state that I charge 10% with a minimum of €40.

How do I do this correctly from an accounting perspective? What do I do with the first invoice? And what about the second one?

Thank you for your prompt answer. Best regards, Sam