Car air connditioner repair?

Question

I went to the mechanic to fix my air conditioner which was blowing warm air, which sucks during a Texas summer. The mechanic fix the air conditioner by changing the air conditioner belt and an added bonus they change the alternator belt (sarcasm).
Couple of days later the car start making a really awful screeching sound, i take it back to them and they say that my bumper is not strong enough for the belt, so they say that the bumper needs to be changed at an estimated charge of 300 dollars.
The air conditioner belt that they changed, snapped today in the highway, only after one week of them changing it.
So i'm 177 dollars down, with no AC and with a car that makes an awful screeching noise which makes people turn heads a block away. What would you advice me, are this mechanics being extremely negligent and don't want to own up to their incompetence or is there any other explanation.
Thanks