I deal with this issue a lot during the C.A. phase of my projects. All contractors have to mark up prices for specified items they provide. The mark-up can run anywhere from 10 to 30%. The discounts they get also vary widely. It is fair to compensate them for the time it takes to order the product and get it to the job site when they need it if you expect them to take responsibility for delays, misordered merchandise, faulty products, etc. If you've ever done this yourself, you would understand. Quality control is atrocious these days and somebody, somewhere always screws up something along the chain of custody.
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A. Atkinson
A. Gordon Atkinson, Architect
San Francisco CA
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