Hi, I may be wrong on this, but I think that if you add a stock item to an invoice, it then decreases your stock amount by what you've added to the invoice. Isn't this what Wright was after?
I think it's what we need, and it sounds similar - we keep a certain number of items in stock as spares, and when we need to supply them to a customer, or use them in a repair job, we add them to that customers invoice. Sadly, what that doesn't do is add the cost of the stock item to the customer's project - that would be my feature request please!
When we get down to a certain level, we simply order more from our supplier, and put that cost against general Project Costs rather than any specific customer project.
I hope that helps Wright, although it's been some time I know, as I think that'll help you keep track of your stock, even if not your specific project costs.