From the "Halacha for Today" on Thursday, November 15, 2012 (emphasis mine):
If one receives a letter or an invitation that includes a return envelope with postage already affixed to it there is no obligation to respond to the letter or invitation just because the postage was pre-affixed, and there is also no obligation to return the stamped envelope to the sender
However, it is prohibited to use the stamped envelope for another purpose; it should be put aside until the arrival of Eliyahu Hanavi. (Ruling of HaRav Nisim Karelitz Shlita quoted in Sefer Hashovas Aveida K’Halacha page 116)
. By R' Feinhandler. Read first 40 pages for free. Find in a library near you. Not available on <hebrewbooks.org>.