Which revolutionary retail concept did the U.S. military introduce during World War II that fundamentally changed how products reached frontline troops?

The Post Exchange (PX) system has been a cornerstone of military life for generations, providing essential goods and a taste of home to service members worldwide. These military retail establishments have evolved significantly over time, adapting to changing warfare conditions, technological advances, and shifts in military deployment strategies. This trivia question explores a fascinating aspect of military retail history that transformed how supplies reached troops in combat zones.

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Mobile PX trucks ("Rolling exchanges")
Automated vending machines at forward operating bases
Mail-order catalogs with battlefield delivery
Self-service commissaries replacing quartermaster distribution