Driving from San Diego to Baltimore usually entails a trip through the Rockies. I’ve seen many people who use up more of thier brake pads going once through the Rocky Mountains than they normally would in a year of driving. However, I have noticed that pads typically last more than 2-3 years. I think you may have purchased an inferior brand of brake pad at the last change out. Also, if your rear pads are not at 2 mm yet, it’s quite normal for the front pads to wear more than the rear since most cars get 80% of thier braking power from the front brakes. what I think happened is that you used more braking through Rockies, and where even I tend to ride the brakes even on the interstates, and a combination of cheaper brake pads. I really don’t think that the mechanic in Baltimore is going to claim something that cheap to fix and have it be a scam.