AC troubleshooting

I'm new to this, so bear with me. Recently on a long road trip I stopped at a rest area for ten minutes and when I turned my car back on the AC suddenly was not working. It went from blowing perfectly cold air to hot/humid air in a very abrupt transition. Air still blows. Temperature appears to work. Button lights up when I press it. AC is just not functioning.
I brought the car into a Toyota service center. After nearly three hours (and nearly $300), I was advised that they were not certain where the issue was but after ruling out leaks or problems with the compressor the issue may be the AC pressure switch. I was told the part would be $320 and would require evacuating and recharging the AC system in order to replace it ($220). Bonus, they did not have the part so I would need to come back.
I was taken aback at the cost of "we're not sure but this might be it, we won't know until you spend $850 and can't guarantee the issue will be fixed" and am worried I'm being taken advantage of. I'm having trouble researching online (was given no further information than "AC pressure switch"), but it appears I can find the part at 1/10 the price?
Does anyone have any insight into this, or suggestions for good places to find information? Feeling very in the dark as the associate could hardly articulate the problem or answer my questions.
Thank you!
(2010 Prius)

 

They did not say. Initially they told me there was "abnormal voltage" but then they said they could not locate the issue with certainty. I was hoping to get some more information if I called back on Monday. It does say "AC pressure switch" on the pre-work order they printed me, but no product codes or any further descriptors.

 

What diagnostic codes did they get from the system, did they say?[/QUOTE]
They did not say. Initially they told me there was "abnormal voltage" but then they said they could not locate the issue with certainty. I was hoping to get some more information if I called back on Monday. It does say "AC pressure switch" on the pre-work order they printed me, but no product codes or any further descriptors.