Customize invoice creation days (NET days)

[Effie]

Hello, I am trying to figure out how to implement the button "Invoice Selected Items" which is in client's summary tab (admin area).  We are in need to choose for some of our customers the days their invoices will be created before the payment's due date.   I was thinking to create a new module where we will be able to type the client's id and the NET days , checking our db for the due date of all of client's services and check if based on our chosen NET days the invoices need to be created. The issue here is that those invoices will be printed again using the default NET days.  I had a ticket for this issue with whmcs support team and they advice to step in and  cancel the invoice before generation to stop the email from being dispatched using a hook point. I don't really like this option. 

Would appreciate your thoughts on the matter