Which retail innovation was FIRST widely implemented in U.S. military exchanges (PX/BX) during WWII before becoming standard in civilian retail?

Military exchanges (PX/BX) have evolved significantly to serve military personnel and their families. These retail operations have a unique history and have pioneered retail innovations that later spread to civilian markets. This trivia question explores how military retail operations have influenced modern commerce and shopping experiences for service members throughout history.

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Self-service shopping format (customers selecting their own merchandise without clerk assistance)
Price matching guarantee for deployed service members
Barcode scanning at checkout registers
Online ordering with base delivery service