On a US military base, the PX is the shop. Soldiers and their families buy food, clothes, soap, snacks, electronics — ordinary life, not weapons. The letters come from post exchange: post here means the base, and exchange is an old army word for that shop. Prices are often lower than outside because of tax rules on the base, and you cannot just walk in if you are not allowed on the base. Air Force bases sometimes say BX instead; the Navy often says the Navy Exchange. In films set on American bases, someone will say they will be at the PX. Taiwan students already know 福利社 from school; this is that idea, but on a US base, and the letters PX are the American name.