For what it's worth, my local post office has what look like metal detectors too, but they actually respond only to specially sensitized magnetic strips included on some Post Office products, like packets of stamps - just like the DVDs and such you purchase at Best Buy.be real careful if you do carry in the Post Office as some have metal detectors.
If "metal" set them off like airport detectors, everything from pens to shoes with metal insoles & steel toes to fancy belt buckles to the Parcel Post package with a cast iron frying pan for Aunt Ginny would be setting them off constantly.