Which retail innovation was first widely implemented in military Post Exchanges (PX) during World War II that revolutionized how service members shopped?

Military Post Exchanges (PX) and Base Exchanges (BX) have a fascinating retail history dating back to the 19th century. These military department stores evolved from simple sutler wagons following troops to sophisticated retail operations serving military communities worldwide. This poll tests your knowledge about a significant retail innovation that transformed how military personnel shopped on bases during a critical period in American military history.

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Self-service shopping (allowing customers to select their own merchandise rather than requesting it from clerks behind counters)
Military payment certificates (MPCs) replacing regular currency for all PX purchases
Catalog ordering systems for troops to request items from stateside department stores
Automated inventory tracking using punch card systems