"As for the analogy about the price of a coke and a hot dog from an entertainment or sporting venue,"
The word there is convience. Convience is worth something.
On the otherhand ...
I expect an electrical contractor to keep an inventory of common materials in his truck. The cost of maintaining that inventory should be close to 5% not 100%. Considering his discounts from retail, I would expect to be charged RETAIL for common materials.
On special purchase materials he is entitled to charge for HIS time to find and deliver the materials.
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On long term jobs I would never hire any contractor who is going to charge above retail for parts.
On short term jobs there is some convience factor.
