In this section, you may add late fees automatically to each invoice. Choose whether you'd like to add a flat fee to the invoice or a percentage of the total as the late fee amount. Next, enter the number of days to pass before the late fee is added to the invoice. For example: If you enter "10", the late fee will automatically be added to to the invoice 10 days after it was initially sent to the customer. Finally, if the Monthly recurring late fee option is switched "ON", the late fee will be automatically added to the invoice every 30 days until it's paid.
For more information, see: Late Fees