Put a regular stamp or two on it and mail it at least a couple of business days before June 7th.
On May 19th 2016 I called the registrar of voters' office to ask what would happen if someone mailed in their ballot with insufficient postage: they replied "It would be taken care of.". See this article from 2008 http://www.sfgate.com/bayarea/article/Ballots-with-incorrect-postage-to-be-delivered-3264761.php
The Alameda County Registrar Office will be charged for the postage due by the Post Office. One more reason to implement internet voting.