Which retail innovation was first widely adopted by military Post Exchanges (PX stores) during WWII before becoming standard in civilian retail?

The Post Exchange (PX) system revolutionized retail for military personnel and influenced civilian retail strategies worldwide. These military department stores provided soldiers access to goods at reasonable prices while deployed and introduced innovative retail concepts that later spread to civilian markets. Test your knowledge about this fascinating intersection of military history and retail innovation that shaped modern shopping experiences.

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Self-service shopping format (customers selecting their own merchandise)
Loyalty discount cards for repeat customers
Online catalog ordering system
Membership-only shopping model