tell them what youre looking for on the way in, theyll point you in the right direction of the car/cars with the bits you want, then you help yourself and pay on the way out
if youre not happy taking it off yourself you can ask them to do it and theyll just stick an extra few quid on.... but if youre not confident in taking it off, then its IS a good idea to do it, because itll teach you how to fit it on your car
so you might as well go and try... and if you break it you can just bugger off and leave it