I know this is an old item, but I just got my 'vote-by-mail' ballot for a special local election and noticed that it included a postage-paid return envelope, which I had never seen before, so I checked the internet for more info and found this:
You won’t have to put a stamp on your mail ballot starting in 2019
http://www.sacbee.com/news/politics-government/capitol-alert/article215115160.html

OCU