Which retail innovation was first widely implemented in military Post Exchanges (PX stores) during WWII, later becoming standard practice in civilian retail?

The Post Exchange (PX) system has been a crucial part of military life for over a century, providing retail services to military personnel worldwide. These unique retail environments have evolved from simple trading posts to sophisticated retail operations, offering everything from essentials to luxury items. This poll explores how this unique military retail innovation has shaped both military culture and broader retail practices throughout history.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Mail-order catalogs with overseas shipping guarantees
Price matching with civilian retailers
Membership cards with purchase tracking