There are so many times clients come back with "I'd like to add this" or "Can I upgrade that?" but there's already a payment plan for the work. I'd love an easy way to add whatever the new service is to the invoice without having to make a note of it then, when the project is completed, finalize the payment plan and immediately have to go into the invoice, return to a draft, update with whatever the additional services are then resend. I have tons of notes all over the place, and I'm sure I'm forgetting to update people's invoices when we finally finish the project.