Which groundbreaking retail practice was first widely implemented by military exchanges (PX/BX) during WWII before becoming standard in civilian retail?

The military exchange system has been a unique retail operation serving armed forces personnel for generations. These stores, known as PX (Post Exchange) in the Army and Air Force, NEX (Navy Exchange) in the Navy, and MCX (Marine Corps Exchange) in the Marines, have a fascinating retail history that has influenced modern commerce in surprising ways. Test your knowledge about this unique intersection of military history and retail innovation!

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Loyalty reward programs for frequent shoppers
Price matching guarantees
Seasonal merchandise rotation