Which retail innovation was first introduced to American military PX stores in 1949, revolutionizing how service members shopped during the post-WWII era?

The Post Exchange (PX) system has been a cornerstone of military retail for over a century, providing essential goods and a taste of home to service members worldwide. This innovative retail concept has evolved dramatically over time, from simple sutler wagons following troops in the field to modern department store-like facilities on military bases. Test your knowledge about this unique military retail innovation that has shaped the lives and morale of countless service members throughout history.

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Self-service shopping (converting from clerk-service to customer-selected merchandise)
Military payment certificates (MPCs) replacing cash transactions
The Army & Air Force Exchange Service (AAFES) unified brand system
Catalog ordering for troops stationed at remote outposts