vTiger 5.4.0 Modify Invoice Module

In Invoice module we need to have so when customers invoice is over due it automatically sends Reminder Invoice and if still no payment within 10 days it sends a second reminder invoice with Reminder fee and interest rate. We use PDFMaker and Emailmaker from its4you so its has to be compatible with this contributions In admin, we need the ability to set reminder fees and default interest rate and how many days the invoice should be over due before sending the first reminder and second reminder. when the reminder is sent, it should automatically calculate the amount the customer shall pay and make the Reminder Invoice with the extra fields of costs and recalculate the total amount on the invoice. We have also made several modification on this Invoice module so it is not original Please be free to ask questions so you know what the task is about