Which retail innovation was first widely implemented in Army Post Exchanges (PX) during WWII before becoming standard in civilian stores?

The Post Exchange (PX) system has been a crucial retail innovation for military personnel since its inception. These stores have evolved from simple sutlers' tents during the Civil War to sophisticated retail operations serving troops worldwide. Beyond providing essential items at discounted prices, PXs have influenced civilian retail practices and served as testing grounds for retail innovations. This poll tests your knowledge about this unique military retail institution that has supported troop morale for generations.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Point-of-sale computing systems for inventory management
Branded store credit cards for military personnel
Standardized price labeling across all store locations