I walk in and sign a form that states I agree to pay diagnosis fee not to exceed ~$120. Diagnosis happens, I get an estimate of the work required to execute the repair. I can't recall a time when I paid anything other than precisely the amount of the quote offered ahead of time. Maybe it happens but it's vanishingly rare.

Best part is that the repair center will triage and rank the problems and often offer more than one treatment which will vary on endurance/cost/etc. Would that medicine could offer the same!