In 1983 , the U.S. Postal Service began using an expanded ZIP code system called `` ZIP + 4 '' , often called `` plus-four codes '' or `` add-on codes . '' In 1983 , the U.S. Postal Service began using an expanded ZIP code system called ZIP +4 , often called `` plus-four codes '' , `` add-on codes '' , or `` add ons . ''