For Jobs where we've included some number of extras to account for St. Murphy Events, our procedure is:
1) At the outset identify if the customer is willing to buy the extras, if any, when the job is completed.
2) If yes, they ship with the order.
3) If no, they are boxed up, labelled, and put into our special "Extras" Inventory stock. Should the customer have an emergency and needs something fast, we've been able to ship them a few parts instantly, or a jump-start on a repeat order.
We might also put parts on this shelf if they are . . . slightly . . . out of tolerance. Again, for the purposes of a rapid response to a customer in the event they have a problem. We've actually had this happen a couple of times, we simply let them know what's being shipped and what the dimensions are, and we've been able to save a customer some heart ache by keeping them running long enough to get an order of conforming parts to them.