so is a building package in the digital module issue, i think we have found it and corrected , download again please.
if a invoice are now a due date and is subject to a late fee, the plugin when detect it, late say the invoice id 520 and the invoice total of 100$, you have set a 20% percent as a late fee amount, then the plugin will do the fallowing :
Create new invoice = a new invoice with new ID will be created with the amount of 20$
Add late fee to invoice = the plugin will add 20$ to the invoice ID 520, the the invoice ID 520 will have a total of 120$
i hope this is clear now .