Which wartime innovation permanently transformed military retail when it was first implemented in Army Post Exchanges (PX) during WWII?

The Post Exchange (PX) system has been a crucial part of military life for generations, providing essential goods and boosting morale for service members worldwide. This unique retail environment has evolved significantly through major conflicts and peacetime alike. Test your knowledge about this distinctive military retail institution that has shaped soldiers' experiences and created a specialized retail ecosystem unlike any civilian counterpart.

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Self-service shopping layouts (allowing customers to select their own merchandise)
Military payment certificates instead of regular currency
Commissary credit cards for military personnel
Standardized pricing across all global installations