I’ve been to mechanics. I’ve pretended to know what’s going on. I’ve been blind-sided by an actual bill that’s several times larger than what I had expected.
This article makes several great points about expectations management. Transparency and honesty improve customer satisfaction over time, even if they bring bad news. Disagreement is a part of that. When a customer comes to you with an expectation that you disagree with, you need to set them straight. If I’m the customer, I’d rather be informed and educated about the incorrectness of my assumptions than be upset and disappointed after the work is done.