[nandomrumber]

What is an appropriate fee to charge any particular customer when what you’re billing for isn’t necessarily time and materials, but more so the skill / experience required to diagnose and rectify an issue, or otherwise provide a solution, in a timely manner?

I’d argue the most correct answer is: up to the customers ability to tolerate, such that they’d be inclined to want to become repeat customers.

[dataflow]

I'm not asking what's appropriate. I'm asking what is legally obligated.

[rcxdude]

The legal standard will be related. Absent a written deal, the court would look at what the customer would have or should have reasonably expected, based on typical rates for such work, the value and cost of doing the work, etc. It can get quite messy and usually it's not worthwhile litigating such a thing.

(heck, even with a written deal, the fine print often matters less that people might think in court, though a badly written contract will sure draw out much more protracted arguments about such details)

[pasc1878]

Depends on the contract the parties agreed on.