2001 Town and Country A/C compressor?

2001 Chrystler Town and country, 100,200 mi, Automatic. 2 wheel drive 4 cylander.
This car was given to us by my in laws, they maintained it well. About 3 weeks ago the airconditioner was blowing just plain warm air, we took it in and they said it was down 2lbs of coolant.
When we brought it home it cooled fine again. Over the past 3 weeks it has been cooling less and less. It was gradual so we didn't notice right away. I suspected a leak so we took it back and asked them to check it out. Now they are saying that the compressor it out and it is going to cost $900 to fix. I read somewhere that overcharging the system can cause the compressor to go out. The place we took it to we feel is trustworthy--however it is too coincidental that this happens only 3 weeks after they charged it. When I took it in it was still blowing cool air but it wasn't super hot outside. Also was wondering if maybe they didn't put the correct kind of coolant. Basically hoping that there is another alternative to paying a repair bill of $900!
Thank you!