Which retail innovation was FIRST introduced to military Post Exchange (PX) stores during World War II that revolutionized shopping efficiency for troops?

The Post Exchange (PX) system has been a crucial part of military life for generations, providing essential goods and services to service members worldwide. These military retail operations have evolved significantly over time, incorporating cutting-edge innovations while maintaining their core mission. Test your knowledge about how these unique retail establishments have supported armed forces and adapted to changing military needs throughout history.

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Self-service shopping format (allowing customers to select their own merchandise)
Military payment certificates (specialized currency for overseas bases)
Mobile PX units (truck-based retail for frontline troops)
Catalog ordering system (for remote outposts)