How do I send an updated invoice that properly communicates w/ customers? When I add a line item after it sends full invoice again instead of new small amount.
I'm happy to shed light on why the entire invoice is sent to your customer.
An invoice is an official document of a transaction between you and your customer. Therefore, your copy in QuickBooks should be a replica of their invoice. If you need to edit and send it to them, they will have to see the entire transaction with your changes. This will prevent you from having problems when authorities review your records as well.
If a customer owes you again, you can send them another invoice instead of editing the old one. Then, you can send them a statement, listing all their outstanding invoices.