Dezwitinc: I agree totally. I charge for every bit of work done. Things come up all the time. So after a fair price including phone time, scheduling, labor, material, overhead & profit etc., I was assuming the question was about adding an additional amount to the billing. So if the customer asked what was that for I would have to say " mark up I lost on the gen you didn't buy from me cuz I got cut out of the deal " and that would get uncomfortable. I don't make it punitive to the customer for not buying it from me but I don't mash my schedule to fit them in if I don't have time.
Handling, phone calls, setting in place, missing parts, all of that is charged under T & M.