Which retail innovation was first widely implemented in military Post Exchanges (PX) during World War II that revolutionized how soldiers shopped and later influenced civilian retail?

During major military conflicts, retail operations within military bases (Post Exchanges or PX systems) have undergone significant transformations to serve personnel better. These innovations often pioneered retail concepts that later became standard in civilian markets. This trivia tests your knowledge about a groundbreaking retail innovation that emerged from military necessity and changed how soldiers shopped during a critical period in military history.

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Self-service store format (allowing soldiers to select their own merchandise rather than requesting items from clerks)
Military payment certificates (specialized currency) with embedded RFID tracking
Computerized inventory management terminals at each PX location
Automated vending machines with credit card capabilities