Our invoices always carried a parts list, pricing plus labor, tax and all. If they ask me if that is what I paid for the parts, I used to say, "Those prices are exactly what you would have had to pay for them unless you went used or outdated. Since I have to guarantee my work, I can't go with used or outdated equipment. All has to be factory fresh. That usually stops any further inquiry and if not, I then said, "You have me at a disadvantage with pricing. I don't do the buying, just the installations." If it does go any further, one can try to explain that "if you were to buy 15 of these condensing units, you can get a discount too but it won't be as much as you'd like."