Which famous retail innovation was first widely implemented in Army PX stores in the 1920s, decades before becoming standard in civilian retail?

The Post Exchange (PX) system has been a crucial part of military life for generations, providing troops with essential goods and a taste of home while deployed. These retail establishments have evolved dramatically over time, from simple commissary wagons following troops in the field to modern shopping complexes on military installations worldwide. Test your knowledge about this unique retail history that has supported military personnel and their families for centuries.

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Self-service shopping (customers selecting their own merchandise without clerk assistance)
Installment payment plans for military personnel purchasing expensive items
Computerized inventory tracking systems
Mail-order catalogs with delivery to remote outposts