I use the sales order process to track what jobs I still need to complete and send out, and which ones are complete. 

I have items that are not tracked in my inventory, but are NOT listed as service items, and are not being manually typed in to the sales order. I pick them from the list of items. 

As soon as I then send the invoice (we create custom apparel so we pre-invoice the work before completing it) the sales order is marked as completed taking it out of my workflow. 

How can I change this behavior?