Which famous retail innovation was FIRST widely implemented in Army Post Exchanges (PX stores) before becoming standard in civilian retail?

Military exchanges (PX/BX) have a fascinating corporate history dating back centuries. These retail establishments have evolved from simple sutler wagons following troops to modern department store-like facilities serving military communities worldwide. They've survived wars, technological changes, and shifting retail landscapes while maintaining their unique position in military culture. Test your knowledge about this distinctive retail institution that has supported troops throughout military history.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Point-of-sale computerized inventory tracking systems
Barcode scanning at checkout registers
Store credit cards with military ID integration