Blower motor relay replacement

Mechanic told me that AC compressor was bad and there was no use charging Freon when my AC started blowing hot air that it was the compressor. How did he figure that out? Compressor was not making any noises. He wanted $900.00 to repair with $430.00 for compressor itself. I can order a Delphi for much cheaper. Saw all of your cooling system literature about system, compressor replacement and parts tips which were good. Compressor part looked like it could be doable for me. Mechanic was at a Mr Tire. Where different guy had first told that they would start out charging AC then could diagnose any other possible problems. Does that sound right? Yes, I have trust issues with car mechanics. Please advise.

Okay, car first started putting out cold air but a couple months go blower would only work on high. Then all of a sudden last week it started putting out only hot air. Naturally I was hoping it only needed a charge of Freon. That is when the mechanic told me it was compressor which he would fix for $900.00 and the compressor it self would cost $430.00. Found new Delphi on Amazon for $159.00. Have a friend who knows what he is doing. Going to replace the whole thing because there is a lot of stuff I have to take off to get at it. Car has 130,000 on it. I hate to put a clutch on it and then have the whole thing go up. Thanks a lot, you guys perform a great service.