It looks like the lyrics sung in the low voice are from John/Juan himself, while the others that address him are from a neighbor or co-worker. His name seems to be a riff on the notorious made-in-the-USA phrase "gone postal", and he shows all the signs of someone prepping to become the latest mass shooter.
He's an ex-con who claims to have put aside his former violent ways. But he's very touchy about negative online reviews, and he can sometimes be found muttering to himself & standing on the road in the rain. He won't look you in the eye (keeping his gaze hid). He's "missing one stamp" the way we say someone is "one card short of a full deck". He has a diary or a blog where he compiles his grievances and conspiracy theories (his "screed"). One suspects that he has a stockpile of military-grade weapons.
You're afraid John Postal has the potential to do something horrific... and he knows where you live. Finally he is "staring so loud" at you, there isn't any doubt. In the end I believe John Postal goes on his shooting spree (he is "pushed past the brink" & the "postage is due") and finishes it in the typical way: by putting a bullet in his own brain. Thus he goes down the drain, out to sea, upstream, on a wave. He lives out the "same fever dream" that has driven dozens of other lonely American men down this path. --Nehushtan (talk) 09:09, 15 June 2020 (EDT)