one reason they do that is because they don't want "come backs"....
Whenever getting shop work done add about 20-25% to the quote. This way you are not disappointed when they throw the extra parts in or you see the extra charge for having to
replace worn or broken parts to get to the main
issue.
Sometimes I'll add a little padding on maybe a headgasket or engine job......but otherwise I just get a good estimate together, if I pad it too much I'll scare em off to another shop, but if I do run into a surprise, then of courses you have too call em back.