If you have time, you can mail your ballot early from your local post office. Make sure it is postmarked to show the mailing date was on or before Election Day. Double check that it is addressed with "USA" on the election office address - sometimes the pre-printed ballot return envelopes are missing "USA"!
The Military Postal Service provides FREE Express Mail ballot return from every military installation. This is a safe and efficient ballot return program for all uniformed service members.
Do not rely on the US Embassy or Consulate to send your ballot back in time. They are not a mailing organization. There may be some assistance through American Citizen Services, however, it varies depending on which country you are in. The best is for you to use Express Your Vote or to send your FWAB from a post office if you are sure you have enough time for mail delivery.