Which military retail innovation was first widely implemented during World War II and revolutionized how soldiers accessed basic goods in combat zones?

Post Exchange (PX) stores have been a crucial part of military life for generations, providing soldiers and their families with essential goods and a taste of home while deployed. These retail operations have evolved dramatically throughout military history, adapting to changing warfare, technology, and soldier needs. This trivia question explores an interesting aspect of military retail operations that transformed how armies supplied their troops in the field.

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Mobile PX trucks that followed combat units to provide on-demand supplies
Scrip-based payment systems replacing cash in forward operating bases
Radio-order catalogs allowing soldiers to request items delivered by supply planes
Automated vending machines installed in battlefield bunkers and command posts