Bike shops need to make money.
They are probably no different than a car garage (and we ALL know what a rip off garages can be - I have some CLASSIC stories...)
DIY all the way if you want to avoid all of these potentially expensive pitfalls.
Garages, plumbers, builders, solicitors, bike shops...........
Don't trust any of 'em!
What gets me are the high-end bike shops that look at you like dirt when you want to buy a small spare part to fix your bike:
"You want to buy what
? You mean you're not here to spend Â£x000 on the Newfadhyperbolx? Get out of my way you insignificant backward pleb."
Either that or ignore you completely until you p*** off.