IU Editorial Style Guide, Letter Z
Although the word zip, in this case, is an acronym (standing for Zone Improvement Plan), we do not write it in all capital letters. Our preference is shared by many other style guides and extends to many other words that began as acronyms (e.g., scuba, radar).
Use the correct four-digit zip code extension whenever possible. The preferred address format is city, two-letter state code, and zip code all on one line. Note that just one space separates the state from the zip code.
Bloomington, IN 47405-1234
See also addresses.