Yes. Even though you will receive an envelope (or printable envelope template) along with your absentee ballot that says "OFFICIAL ABSENTEE BALLOTING MATERIAL - FIRST-CLASS MAIL - No Postage Necessary in the U.S. Mail - DMM 703.8.0," this is not the case for most overseas voters. You must affix correct international postage.
Exception: The postage is prepaid ONLY if you are able to get your absentee ballot envelope to a U.S. Military post office (APO/FPO). Otherwise, it is not free to send it by normal international post.