Which famous retail innovation was first widely implemented in Army PX stores during WWII before becoming standard in civilian stores?

The military exchange system (PX/BX stores) represents one of history's most unique retail success stories, bringing familiar products to troops worldwide for over a century. These stores evolved from simple supply wagons to sophisticated retail operations with billions in annual revenue. They've influenced civilian retail with innovations in global supply chain management and customer loyalty programs. Test your knowledge about this unique retail institution that bridges military history and commerce.

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Self-service shopping (allowing customers to select their own merchandise without clerk assistance)
Layaway payment plans for expensive items
Mail-order catalogs with delivery to remote locations
Electronic inventory tracking systems