This sounds like a good idea but for simplicity and proper record keeping sake, every order must have it own invoice. This is what is done by most organisations with a well structured financial accounting system. If you are following a proper procurement or sales work flows, combining orders on a single invoice will mess up your procurement or sales documentation.
Manager may soon have an order tracking feature embedded in a workflow’ Order => Invoice => Delivery note or Order => Delivery note => Invoice for sales and Order => Invoice =>Goods Received or Order => Goods Received => Invoice for Procurements.
Combining different orders in one invoice will make it hard for the system to know fulfilled orders ( especially if the order tracking is implemented) since only one order number and associated text in other fields can be entered on the invoice , unless a new way of doing things is introduced.
If you don’t care about keeping track of every order in Manager, you can simply add lines and enter all orders on the invoice as suggested by @Tut above. Or deactivate the order tab completely since it won’t serve it purpose.
@Frankie wrong, @lubos is very interested in our discussions, this platform exist for that purpose. With discussions new ideas come up and we can actually come up with better cases for the implementation or not.