While this workflow is currently not as slick as we would like it to be it should still be a nice time saver for you in certain circumstances.
The prerequisites for this are that the invoices you want to modify are in a draft state and that you have the item or discount you want to add already set up under
Invoices > Items & Discounts.
Tip: you can revert invoices to draft again once they are open. You can do this from the Invoices > Overview page or on an Invoice's individual page.
This 3 minute video shows you how it's done. Please contact us with any feedback or questions.