The invoice template with a late fee is for an invoice that was not paid on time and a penalty has been added to the amount. This is common for rent or any product that has not been paid in a timely manner. The invoice should detail the late fee amount, including the calculation, on its own line item. If the invoice is not paid, with all fees and penalties included, the receiving party may be subject to collections.
This is usually the last letter or invoice before the amount gets forwarded to collections.