Which major retail innovation was first widely implemented in Army Post Exchanges (PX) during the 1940s, years before becoming standard in civilian retail?

The Post Exchange (PX) system is one of the oldest and most significant retail operations in military history, providing essential goods and services to service members worldwide. These retail establishments have evolved dramatically over time, from frontier trading posts to modern department store-like facilities, and have played a crucial role in maintaining troop morale and readiness. This poll tests your knowledge about this unique military retail operation that has served millions of military personnel and their families throughout history.

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Self-service shopping (allowing customers to select their own merchandise without clerk assistance)
Credit card payment systems (precursor to modern charge cards)
Standardized pricing across all locations (eliminating regional price variations)
Inventory barcode tracking (early version of UPC system)