What do the following phrases have in common?

ABSOLUTE ZERO
CLASS ACTION
MOTION SICKNESS
SPECIAL DELIVERY
FOREIGN LANGUAGE
MOURNING DOVE
RIGHTEOUS INDIGNATION

HINTS:
#1: It has to do with geography.
#2: It has to do with US geography.
#3: It has to do with initial letters.

Answer:

The initial letters of the two words in each phrase form the postal code of a US state:

AZ - Arizona
CA - California
MS - Mississippi
SD - South Dakota
FL - Florida
MD - Maryland
RI - Rhode Island

Credit: Idea - LA Times Crossword
Sylvia Bursztyn and Barry Tunick
GOING NON-POSTAL