Which innovative retail practice was first widely implemented in military PX stores during the 1940s before becoming standard in civilian retail?

The Post Exchange (PX) system represents one of the most unique retail operations in the world, serving military personnel and their families across global installations. This retail network has evolved from simple sutler wagons following armies in the field to modern department store-style facilities with billions in annual sales. Test your knowledge about this distinctive military retail operation that has influenced both military morale and civilian retail practices.

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Self-service shopping (allowing customers to select items themselves rather than requesting them from clerks)
Discount pricing based on volume purchasing (the precursor to wholesale club models)
Store credit cards with military pay deduction options
Standardized store layouts across geographic regions