Yeah, I ran into that all the time when I worked for a service company. Our trucks were stocked with an insane amount of parts inventory to the extent that we could do a typical 200 amp service change on the spot. We carried somewhere in the area of $10K worth of inventory per truck. Still, it was inevitable that we'd encounter some strange doohickey that we didn't have.
Our company's policy on this was really quite simple: The customer could pay us for our time to make a special trip to the supply house or we could order it and return at another time to replace it. In almost every case, they would opt to just pay us to go get it since they had already taken off work to be there. More importantly, whatever we were there to fix was critical and couldn't wait anyway, like a heat sequencer, etc.
