How the heck can mail-in voters be expected to figure out the correct postage if they don't have postage scales? Most post offices don't even have stamp machines anymore (the PO laid off the machine repair people) so you have to stand in line for half an hour each time.
The Post Office will accept and deliver all election related mail regardless of insufficient postage.
Footnote to this, the Postal Service regulations require every Registrar to put the words on the ballots about "postage required" even though the Postal Service policy is to accept and process irrespective.
This afternoon I went by the Registrar's office.
"Anything the Postal Services brings to us, we will accept."
When asked how much postage a local Oakland voter should put on a Democratic mail-in ballot: "We have no idea. Every ballot is different."
This will probably cost either the Registrar money because the Post Office will probably bill them for shortfalls.
But the overriding goal of mail-in balloting is to encourage voter turnout and participation in government, not to save the County money on postage.
And a percentage of mail-in voters vote that way because they are house-bound and can't get to a post office to weigh the envelope even if they had enough stamps or knew how buy postage online.
Delaying mailing one's ballot because one doesn't know how much postage or don't have any stamps creates significant costs for the Registrar by requiring them to incur higher overtime costs to process the avalanche of mail-in ballots arriving in the final week.
Conclusion: Don't worry about insufficient postage.