Which retail innovation was first introduced in military Post Exchanges (PX) before becoming standard in civilian retail stores?

The Post Exchange (PX) system has been a cornerstone of military retail for over a century, providing essential goods and services to service members worldwide. This innovative retail chain concept has evolved from simple sutler wagons following troops during the Civil War to modern military shopping complexes. Test your knowledge about this unique retail system that has shaped military life and influenced civilian retail practices.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Store credit cards with monthly billing statements
Barcode scanning technology at checkout counters
Store-brand generic products competing with name brands