Which famous retail innovation was first widely implemented in military PX stores during WWII before becoming standard in civilian retail?

The Army & Air Force Exchange Service (AAFES), commonly known as the PX (Post Exchange), has been serving military personnel and their families for over a century. These specialized retail establishments have evolved from simple trading posts to modern department stores offering tax-free shopping on military installations worldwide. Test your knowledge about this unique military retail institution that has played a crucial role in maintaining troop morale and readiness throughout American military history.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Credit card payment systems
Barcode scanning technology
Membership loyalty programs