When I got to the counter, the nice ladies weren't serving me. A more typically "postal" clerk explained that they can't take that much mail over the counter, it has to go "around back".
So I load it back on my rig and go around back. The guy you have to check in with says "Um, er, yeah, but the problem is no bikes." I shot back "This is not a bicycle. A bicycle has two wheels." He laughed loudly. I said "It's a tractor-trailer." He laughed some more and asked incredulously "Why, because of that!?" pointing towards my trailer. "Uh, yeah, this is a human powered mail truck!" After a little bit more talking, he caved and let me in. I told him I didn't know where to go, he told me the wrong place (there are about 100 loading docks), I went there and when I came out, a postal police was waiting for me. He was trying to tell me "no bikes" but I was telling him how they wouldn't take this much mail over the counter and I understood this was very unusual but I would be very careful, etc. He was really nice and escorted me to the correct loading dock, waited for me to unload, then escorted me back to Brentwood Road.
So it worked out this time, but I'm not as excited about the new client and the heavy load. I don't know how this will play out, but our society's bias against human powered vehicles sucks.