In the Invoice Creator select the Project and do a Part Invoice on it. Select in the selector window the Project Work Order called Invoice.
The Invoice will be empty, now add the costs directly to the Invoice via the drop down icon. Try to always add an existings Object from your database this way you can later run a Financial Report on your Objects. You can still rename the Object’s name and the price and add a note.
Check the Project and see the Invoice against it.