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

Military Post Exchanges (PX) have evolved dramatically since their inception, transforming from simple supply depots to sophisticated retail operations serving military personnel worldwide. These unique retail environments have pioneered innovations that later spread to civilian retail, particularly during wartime when resource constraints demanded creative solutions. Test your knowledge about a significant retail innovation that began in military exchanges before becoming commonplace in civilian retail.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Barcode scanning at checkout
Loyalty card programs for repeat customers
Centralized distribution centers for inventory management