Which major US retail innovation was first widely implemented by the US military during World War II to efficiently supply troops with personal items?

The retail landscape within military installations has a fascinating history that many civilians never encounter. Post Exchanges (PX) and Base Exchanges (BX) represent a unique retail environment that has evolved significantly since their inception. This trivia question explores a pivotal moment in military retail history that transformed how servicemembers accessed consumer goods during wartime and eventually influenced civilian retail practices as well.

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Self-service shopping (where customers select their own merchandise without clerk assistance)
Mail order catalogs with photograph-based product listings
Membership-based shopping with ID verification
Standardized pricing across all global locations