Peter,

I would recommend that you demand a record of the work performed before
paying the bill. If they won't go along, then use another shop.

I recently questioned a shop about the amount of time they charged for an
annual. They told me that 2 hours was spent on the paper work. I figured
out that they could not have spent nearly that time much and their write up
wasn't even complete. They just lost a LOT of potential business.

Vote with your feet.