Which retail innovation was first introduced to American military Post Exchanges (PX) during World War II that revolutionized how soldiers shopped?

Military retail has a fascinating history of innovation that has served troops for generations. The Post Exchange (PX) system revolutionized how soldiers accessed goods while deployed. This unique retail model has evolved dramatically from its origins in frontier outposts to today's sophisticated operations. Test your knowledge about this specialized retail system that has been crucial to military morale and readiness throughout modern warfare history.

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Self-service shopping format (allowing soldiers to select their own merchandise instead of requesting items from clerks)
Military payment certificates as a specialized currency
Catalog ordering system for overseas deployment
Rationing punch cards for luxury goods