Why did I receive an invoice even though I have paid the rest of my debt?

The final amount of final balance must be paid by the due date.  

Check the final balance on the invoice for the month in question from onlinebank.

If you have paid the final balance of the debt after the due date, you will still have to pay the following month's costs and interests. In this case, you can deduct the amount you have already paid from the current month's invoice and pay the difference by the last day of the month.