px 便利店

PX refers to the "Post Exchange" or "Base exchange", a store operated by the Army and Air Force Exchange Service
美国军事基地便利店

Please see the following "extract" about its histroy.

"The origins of the AAFES date to the late 19th century, when trading posts were first established on military bases. Until then, itinerant peddlers known as 'sutlers' had served the day-to-day needs of soldiers for such items as shaving gear and tobacco. In 1889 the U.S. War Department officially sanctioned the presence of base canteens, and six years later the department issued General Order 46 establishing a system of base exchanges at virtually every military post. Each military organization had its own system for the exchanges, with post commanders typically assigning an officer to run the unit. The commanding officers decided how to spend the profits. Soldiers soon began to refer to these post exchanges simply as 'PXs,' and t