A little secret about your CA mail ballot: It will get...

Up to two-thirds of Californians casting ballots in the midterm elections are expected to do so by mail. How many will attach enough postage is an open question.

Some counties’ ballots are so heavy that the U.S. Postal Service will demand $1.57 to deliver the envelope. No ballots will come back to voters if they attach only one measly 50-cent first-class stamp, however: The Postal Service delivers every ballot to county elections offices, even envelopes with insufficient postage or no postage at all.

“Our priority is when we see a ballot, we get it delivered,” Postal Service spokesman Augustine Ruiz said in a statement. “We will not deny voters their right to vote by delaying a time-sensitive ballot because of insufficient postage.”

The post office won’t foot the bill, however. County elections offices reimburse the Postal Service for under-stamped ballots.

“They send me a bill every day,” said Scott Konopasek, assistant registrar of voters for Contra Costa County. “Sometimes we get two invoices a day.”

For some of California’s voters, the postage is already taken care of. As of July, 10 of the state’s 58 counties planned to provide postage ballots for the midterms.

That includes San Francisco. Since 2003, the city has sent out all ballots with postage-paid return envelopes, Elections Director John Arntz said.

When the city moved from punch cards to the larger optical scan ballots, four to five stamps were needed for ballots to be mailed.

“A lot of voters didn’t know or didn’t realize, and a lot of voters didn’t like needing extra postage,” Arntz said.

All counties in California will have to follow San Francisco’s lead starting next year, when a new law takes effect making mail ballots postage-free for voters.

Konopasek estimated that paying for postage will cost Contra Costa close to $500,000.

With or without the correct postage, early voting through mail-in ballots has already begun. Californians have until Oct. 30 to apply to vote-by-mail, either online or at their local county elections office. Ballots postmarked for election day will be counted up to three days after the Nov. 6 election.

Holly Honderich is a San Francisco Chronicle staff writer. Email: holly.honderich@sfchronicle.com Twitter: @hollyhonderich