Which retail innovation was first introduced at Army Post Exchanges (PX) during WWII that revolutionized military shopping and later became standard in civilian retail?

Military exchanges (PX/BX) represent a unique retail innovation that has served military personnel for generations. These stores evolved from simple sutlers' wagons following troops during the American Civil War to modern retail complexes offering tax-free shopping and generating billions in revenue. Beyond mere shopping, they provide a taste of home for deployed service members and their families while generating funds that support important military community programs. Test your knowledge about this unique retail institution that has been part of military life for over a century!

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Self-service shopping (customers selecting their own items rather than requesting them from clerks)
Military-themed mascots and branding to boost morale
Catalog ordering system for remote bases
Combat zone pop-up retail locations