WOW! Thanks for that! This is why I love this forum. I was completely planning to go the other way - start with order, and have a "generate invoice" button. While that plan would make things easier for me (as the db programmer) it wouldn't fit the user's workflow all the time. Your solution, however, starts with the invoice but allows the users to just find all the orders/products outstanding, then select the ones they need. That is too cool! Love to have different brains with different biases/perspectives look at issues because there's always a different, often better, way.
Now you've got me thinking: I could potentially do it both ways. Invoices that only bill one order might be easier to go from the order side, but invoices that bill multiple orders would be easier to do this way. Let the user decide. Now I just need to noodle out how to make recurring invoices simple...