Which retail innovation was first widely implemented in military Post Exchanges (PX) during WWII before becoming standard in civilian retail after the war?

The Army & Air Force Exchange Service (AAFES) and Navy Exchange (NEX) have been serving military personnel for generations with retail services. These military-specific retail operations have evolved dramatically through conflicts from World War II to modern deployments. This trivia question explores how wartime necessity drove retail innovation that would later influence civilian shopping.

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Self-service shopping model (customers selecting their own merchandise)
Mail-order catalogs with standardized sizes
Point-of-sale inventory tracking systems
Membership-based shopping with ID verification