During World War II, mail sent from United States Army Post Offices (APOs) back to the United States or to other APOs was, indeed, free, if special services such as air mail were not required. However, APO mail sent to foreign addresses was subject to taxation, as the cover below shows.
This cover was posted from APO 254 on May 12, 1945, to Rumelange. On that date, APO 254, which was part of the 4th Armored Division, was at Schuttenhoffen, Czechoslovakia. The letter was taxed in Luxembourg at the 20g domestic letter rate “1,20” franc in blue crayon, with the tax paid with a 1.20 franc definitive struck with the “T” mark.
Thus, the cover is distinctive, both as APO mail and as insufficiently paid incoming mail to Luxembourg.
Numerical Listings of APO’s January 1942 to November 1947, Army Postal Service and Strength Accounting Branch, AGO, retrieved December 8, 2012, from http://www.7tharmddiv.org/docrep/Location%20of%20APOs%201942-1947.pdf