Service calls, vs. established customers...completely different worlds.
Ms. Yellow Pages calls you and says her living room light isn't working any more....could you come and look at it?
In that scenario, you explain your 'trip charge," go out there, look at things, see what's involved, then stop and talk to her. You tell her the foozewhopper is shot, and the doppleganger needs calibration, that this will probably take three hours, cost $220,- would next Tuesday be ok? Or Wednesday? Oh, and could she get the piano moved, so you can access the attic? Then you charge her for the trip charge, or diagnosis time, whichever is more.
In the other situation, prpperty manager Mr, Reg Cust calls. You go out and look it over, and give him your estimate. You might discuss with him how he wants your bill presented, and adjust your numbers to match.
Now, maybe he just calls and says "do it." T&M can be a very good way to do business- IF there is a well developed trusting relationship between the two of you. The sort of relationship where you don't charge him for that unnecessary second trip to the parts house, and he has no problem paying for delays caused by unforseen problems, or for time you spent in your shop fabricating the thing that took only 15 minutes to install.
"Parts expenses" are one area where that trust can be destroyed. Anyone out there ever hear about the hospital that charged $100 for an aspirin? It just doesn't fly.....the customer isn't going to agree to pay you $20 for a part he can get at the box store for $3.
Add to that other things that run up parts charges....buying a whole new fixture, just for that one piece, or paying for shipping, and there's plenty of room for unhappiness.
