Which military retail innovation was first introduced during World War II and revolutionized how products reached frontline troops?

The Post Exchange (PX) system has been a crucial part of military life for generations, providing essential retail services to service members worldwide. These unique retail environments have evolved dramatically since their inception, offering everything from basic necessities to luxury items. How well do you know the history and impact of military retail operations that have supported troops across conflicts and peacetime alike?

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Mobile PX trucks that brought retail goods directly to combat zones
Catalog ordering systems with delivery to field post offices
Personalized care packages with customized retail products
Rationing credit cards for use at overseas exchanges