Hi John

First, be aware that unless you have a statement in your contracts or quotes regarding interest, many customers will refuse to pay it; hell, some will refuse to pay it even if you have it in there.  Be sure to use discretion on interest charges for the most aggregious abusers of your credit.

Create a new item for addition to an invoice and then when you want to charge a customer interest, edit their existing invoice and place an interest charge on it.