Well I guess, I might want to fully explain the issue with your intended machine shop. Get their direct rationale for why extra money is needed from you when the kit is supposed to be balanced already. I would also ask that if it is within tolerance that your machine eat the cost to double check, or if he is unwilling to do that, then I would suggest you advise him you would expect that if you do pay him to balance and go over they you make the tolerances even more exact. If you are stuck paying for it, you might as well get an even better product out of it in the end. So if the initial quality turns out good, and the machinist still requires additional work, then I would set expectations high for what you want in return.