Which military retail innovation was specifically developed during WWII to boost morale and is still used in a modernized form today?

Throughout military history, the retail experience for soldiers has evolved dramatically. From the early sutlers who followed armies with basic provisions to modern PX (Post Exchange) systems, the retail history of military supply chains represents a fascinating intersection of commerce and combat readiness. Test your knowledge about how militaries have historically acquired, distributed, and sold goods to their personnel across different eras.

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The PX (Post Exchange) system with standardized pricing across all theaters of war
Mobile commissaries known as 'Rolling K-Rations' that delivered hot meals to frontline troops
The Military Scrip System that eliminated cash transactions in combat zones
Battlefield Catalog Orders allowing soldiers to request specific civilian items from home