they are charging you for diagnosing a problem... and they charge a shop rate... how many hours did they spend diagnosing an issue that you TOLD them of? What was the determination? the wheel? bad connection? What exactly did you tell them was the issue?
personally, at my shop, if an issue with a part on a bike is found, I would never charge for the part or repair time... makes for good
PR, eh? Seems to be working fine... my business has become quite busy just by word-of-mouth.... if the customer is at fault, they pay.
please detail the issue found.