Concerning voting by mail, I recall a number of years ago we had to put our thumbprints on checks when we cashed them. We should do the same for any mail-in ballots. On the envelope in which you certify the ballot, the person should put their thumbprint.
This would allow for easy verification if there is a question about an individual ballot, as well as a quick means to audit mail-in ballots. You have to sign the envelope, just add your print to it.