There isn’t a feature that flags it for invoicing as such, although you may wish to have a look at the project tagging feature. You could tag them to a specific tag (e.g. client name or reference) and remove it when you’ve invoiced, or tag the new invoice to the same tag too so you can see what profit you’ve made on it easily.
There’s more info on projects here:
http://community.quickfile.co.uk/t/projects-in-quickfile/8858
Hope that helps!