At an even higher level, like most things in life, mismanaged expectations and poor communication are the fundamentals to satisfaction in general.
In my own experiences, I always clarify the price, or expected price range for a piece of work whether its planned or unplanned, for anything and everything.
For a more relatable example, when my car was with Tony for the fuel system fitting and tune, my P/S pump started to leak. As expected, Tony called me, quoted me a price range to pull and diagnose the pump. Didn't promise me anything about the resolution or price of it but in the end diagnosis was a bad o-ring, so to replace it was a couple bucks more as the pump was already apart. Whilst the additional cost was unplanned, the management of the problem through to resolution met my expectations for a professional and experienced outfit. Furthermore, this was all detailed to a high degree on my invoice and reflected the phone and email conversations we'd had.
In other examples, when outfits don't meet my expectations, I simply don't use them again. I vote with my wallet and that way I find outfits that do align with and agree to my expectations.