If the point of hiring a well established, experienced, reputable painting contractor is to secure the professionalism and trust suggested to be inherent with that choice, then I would EXPECT that professionalism and experience to include the ability to make the proper and correct calculations for labor and materials for a fixed price quote, and there should be NO reason for the contractor to put the cost burden of their miscalculation on the consumer.
Buying a can of premium paint, then bait and switch over to low-line products? Again, complete nonsense. Think about it for a moment; the Painter needs 5 gallons of wall finish. So, he buys one can of premium and the rest 'cheap'? How is he going to hide the 4 other gallons? What's he going to hide it in??? He only has ONE gallon of premium. It's not as though he's going to keep older can labels, they would be covered in paint of a different colour.