AC filters???

While I was at Firestone the other day getting my tires rotated, the guys told me that I was going to need two new AC filters. They wanted almost $150 to do it, because its hard to get to the filters. I Said !@#$ that!

1. How did they see my filters if they are hard to get to in the first palce?

2. Can my filters really be that bad if my car is 18 months old and I almost never use my AC?

3. Can I replace the filter myself? Is it easy?

4. Is this problem serious?

5. Can anybody explain what the hell AC filter are.

Thanks