Wow, no one says you have to get a spare part to a customer tomorrow morning. Just grab one before it goes to assembly line, ship it with the actual completed machines to a distribution center where it will go to the customer from there. There's nothing "new," except at the beginning/end of the chain, just add a few more of those parts to the next order of them. If it will impact operations to take a few from the beginning, some of those spares will have to be delayed. That's ok, tell the customer 6-8 weeks to deliver a spare part. That was pretty standard shipping times when I was younger.