In the automotive industry, you pay what was agreed when the estimate was given. And even then when an estimate is given and work begins, the price can still go up. That is why it is called an ESTIMATE.

There are set standard labor times, that are made from national averages which is pretty much what shops go by; book time. But book time i just a guideline, and is never followed completely.

Once work starts, the price can go up or down. For example internal parts that may or may not need to be replaced, whether or not they shop lets you know if was actually broken or not is the integrity of the shop.

The price can also go up because the work is becoming more and more difficult due to things that the shop has no control over. It isn't the shops fault everything inside an engine bay is rusted to shit, and every bolt is seized. They may work with it a bit, but if its excessive they will and should be able to charge you more for the work. So if the book says 4, and it looks like it will take 6; you have no reason to be pissed at the shop, keep in mind its your car, NOT theirs.

Also as stated before, techs with more experience will obviously work faster and more efficiently to get a job finished. Just because it takes an experienced technician half the time it takes a newer tech to finish a job, you think he's going to take half the pay? You must be smoking boats loads of shit to think you're going to take that money from a mechanic.

I don't think that you have any reason to be upset with this shop, estimates and book time are usually a minimum of what you should and will be charged for a job. It's just the way the auto industry is, don't like it? Fix it yourself, lol. Just my .02