Official Election Mail

I'm planning on mailing my absentee ballot today.  It has to be postmarked by Monday.

On the outside of the envelope provided with my ballot, up near where one would put a stamp, I saw the following logo:

So...  Do I put a stamp on it or do I NOT put a stamp on it?  It is, after  all, Official Election Mail, Authorized by the U.S. Postal Service.   Yet there's nothing that says that postage is prepaid or anything like that.

I decided to call the Elections Office and they told me that I have to put a stamp on it.

It's kind of confusing.  I'll bet a lot of absentee ballots get returned to sender for this very reason.