As the handy chart in Eli's post illustrates, it only takes one stamp to mail in your November ballot in every county this year except for Franklin and Kitsap. But as my democratic experiment from the August primary illustrates, it likely takes no stamp for your ballot to arrive intact.
Elections departments don't advertise this, but the USPS is instructed to deliver un- and under-stamped ballots, postage due. (Anecdotally, I've heard that the USPS sometimes fucks up and returns unstamped ballots, so if you're going to try this for the sake of trying this, vote early.)
So for those of you who complain that your vote isn't worth the price of the stamp, don't use one. Just vote, goddammit.