I do work for companies that moderate or manage the maintenance of department stores and they call me whenever work is needed at one of their stores. They generally request an estimated amount for the work to be performed and issue a work-order with the amount shown. If it exceeds, I too have to get their blessing after providing resonable explanation.
But, more onto the point, if it's things like changing lamps and ballast I either give a per each price or my T&M rates. If it's a larger item, like a transformer or panel upgrade (or similar large jobs), I give a firm quote price and charge for that in full. (Yes, the quote price is slightly larger than T&M)
John C. Harvey
IndCom Electrical Estimates