Transmission rebuild

Just got my transmission rebuilt. Truck still shutters at different speeds and it still bangs into drive and reverse. Mechanic ships out transmission to his guy. He claims they gave me a brand new torque converter. He is trying to blame me for the shuttering, saying that he did not tell them about that. So my response to him was, shouldn't they have found the problem regardless of them being told in advance or not? And plus, what about testing before it is given back to him? He is charging me $1,950.00 for all this and it is still not right.