Along with your postal absentee ballot, you will get an envelope (or printable envelope template) which says "OFFICIAL ABSENTEE BALLOTING MATERIAL - FIRST-CLASS MAIL - No Postage Necessary in the U.S. Mail - DMM 703.8.0".
Note - it is not free to send it by normal international post. The postage is prepaid ONLY if you are able to get your absentee ballot envelope to a U.S. Military post office (APO/FPO) or U.S. embassy/consulate.
If you send your ballot via international mail, you must affix appropriate postage. It is best to take your ballot to the post office and have the envelope clearly postmarked with the date.