I will ask you a really simple question aston do you say to customers i am
going to charge you x amount to do the the job plus cost of parts and then charge them more than what you payed for the parts.
Simple yes or no
I give my customers a price which they are free to accept or otherwise. I make no claims as to cost at all, ever. Is that simple enough for you.
Now here's one for you again, how do you know what the engineer has or will pay for that valve.