How do you add a late fee to an invoice after 30 days?

Go to settings ---> Account and settings ---> left side, click sales ---> choose the late fees option. From there you can adjust the percentage and frequency. after the 30 day mark is auto adds the fees. Saves me personally ALOT of time, also if you are feeling generous for good customers that just may have forgotten you can easily delete the charge by opening the invoice and trashing the late fee line.