Which retail innovation was first widely implemented in military PX stores during the Vietnam War, revolutionizing how deployed soldiers could purchase products?

Post-Exchange (PX) stores have been a cornerstone of military life for generations, providing essential goods and services to service members and their families worldwide. These retail operations have evolved dramatically since their inception, adapting to changing military needs, global conflicts, and retail innovations. This trivia question explores a significant retail innovation that transformed how military personnel accessed consumer goods during deployment.

This is a live public trivia poll on AIPolls.Net. Vote and see real-time results.

Military-specific credit cards that allowed soldiers to make purchases and pay later upon return to base
The AAFES catalog ordering system that enabled soldiers to order from a comprehensive product catalog for delivery to forward operating bases
Self-service PX stores that replaced the previous counter-service model where clerks retrieved merchandise
Electronic point-of-sale systems that tracked inventory and purchasing patterns of military personnel