Since 1986 or so when a postal worker went berserk in Enid, OK, postal has meant berserk. Zip plus four is an escalation. Hyperbole for postal. If you use postal to mean berserk around USPS workers, you may get an intense reaction. Example: Chris went zip plus four on us. Had to put him down with a tranquilizer dart.
ZIP PLUS FOUR
Meaning of ZIP PLUS FOUR in English
Slang English vocab. Английский сленговый словарь. 2012