After reading a couple threads by this gentleman, I would agree that he should sit down and talk to his tech.
I'm not saying that they're intentionally ripping him off, but they might not be familiar with MB systems or diagnosis of these problems. It sounds like the "part's toss" method is taking place.
The AC system isn't really that complicated, when you think about it. There's the compressor, condensor, reciever/dryer, expansion valve, evaporator, and control unit combined with sensors. With some gauges and expertise, I've found that diagnosing AC problems can be done with reasonable precision.
1998 C230 "Black Betty"