Which retail innovation was first widely implemented in military PX stores during the Vietnam War era, revolutionizing how soldiers purchased goods?

Post Exchange (PX) stores have been a vital part of military life for generations, offering soldiers access to goods while deployed. These military retail environments have evolved dramatically over time, adapting technologies and product designs to meet the unique needs of service members. This trivia question focuses on a significant retail innovation that transformed how military personnel shopped during a critical period in American military history.

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Barcode scanning systems
Credit card acceptance for personal purchases
Self-checkout kiosks
Online catalog ordering systems