Which retail innovation was first widely implemented in Army PX stores during World War II that revolutionized how soldiers shopped and significantly reduced theft and inventory management issues?

The Post Exchange (PX) and Base Exchange (BX) systems have been vital to military morale since their inception. These military retail operations have evolved dramatically over the decades, introducing innovative solutions to serve troops worldwide. Test your knowledge about how military retail operations have transformed to support forces in various combat theaters and peacetime deployments throughout history.

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Self-service shopping (allowing soldiers to select their own merchandise rather than requesting items from clerks behind counters)
Military payment certificates (replacing regular currency in overseas bases)
Catalog ordering system (enabling soldiers to order items from stateside retailers)
Mobile PX trucks (bringing retail goods directly to forward operating areas)