I'm sorry that you feel like you did not get a good deal and I have no experience with this mechanic but I have to say that there's a flaw in your logic.
Good European Car Mechanics don't buy from Advance auto parts, or Napa etc. They buy from a wholesaler like worldpac because they know that they won't get crap. Your high street retailers sell plenty of crap with a lifetime warranty which is fine for the DIY'er who can afford to change the part out over and over again, but not for a shop that loses money and credibility every time a part fails.
Furthermore, shops mark up the price of parts because they can't survive on what they charge in labor. The wholesale price of a Bosch alternator is about $125 and the retail price is $330. This is listed on the wholesaler website which tracks retail pricing for this purpose.
This is how it is everywhere, had you have gone to the Mercedes Benz dealer down the road and been a club member, you could have bought parts at wholesale prices. If you took those parts from the parts counter to the service department and asked them to put them on. They would refuse because they would expect to sell you parts for service at different pricing.
More importantly, I think that you're being a little optimistic with your labor time. I just don't see how any flat rate book can suggest that this job can be completed in 30 minutes.
Just visualizing the job:
You have to disconnect the battery
Loosen the alternator so that you can get the belt off
Disconnect the wiring from behind the voltage regulator
Undo the belts and remove the alternator along with the bracket
Separate the old alternator from the bracket
Remove the pulley from the old alternator
Install the pulley onto the new alternator
Mount the new alternator onto the bracket
Mount the bracket back onto the engine
Wire the new alternator up
Attach the belts
Tighten the belts
Reconnect the battery
Include bringing the car into the shop, gathering tools and it's almost impossible to imagine the most experienced mechanic doing this in 30 minutes.
I would expect it to take less then 2.5 hours but again, book rate will pad out the job a little to account for the expectation that the mechanic may not be familiar with it. It's this way because mechanics are paid by book hours. If they do a job that pays 2 book hours, they will get paid 2 hours regardless of whether they do the job in 20 minutes or in 6 hours.
If there is no job, they don't get paid all.
It may not seem fair when you're comparing it to free labor from your father in law but it is what it is. My father is a Cardiologist but I would not expect to walk into Baylor medical center and get an EKG for free
