Firestone Complete Auto Care

My low tire light was on so I asked someone to come out to my work place, pick up the car, fix the tire and bring back to my workplace. They brought an air pump with them and just aired up the tire (said it was caused from change in temperature) and charged me almost $30.00. Well, the low tire light went on a couple days later so I took it somewhere else. The tire had a small screw in it. That place charged me only $17.00 to fix the tire. $30.00 for air vs $17.00 to fix a flat?