Which retail innovation was first widely implemented in military Post Exchange (PX) stores before becoming standard in civilian retail?

The Post Exchange (PX) system has been a crucial part of military life for centuries, evolving from simple sutlers' wagons to modern retail complexes. These military retail operations have not only supported troops worldwide but have often pioneered retail innovations that later spread to civilian markets. Test your knowledge about how military retail operations have influenced broader retail history and innovations!

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Self-service grocery shopping (where customers select their own items rather than requesting them from clerks)
Barcode scanning at checkout registers
Loyalty card programs with tiered rewards
Online ordering with in-store pickup