We have mail voting as a default in Colorado. When you get your license you are registered to vote and opted in automatically. The one piece that might improve it further is if it came with a stamp to mail back. Otherwise you just drop it off at a drive-up ballot box. You can also vote in person if you want. Hardly anybody does it so there’s never a line.
You get text messages each step of the process too. “Your ballot has been mailed”/“your ballot has been delivered”/“your ballot has been received”/“your ballot has been counted - thanks for voting”.
The ballots envelopes (not the ballots themselves) are keyed to the voter's identity. When the ballot is removed (not until the signature is verified and not contested), the voter is counted as voted, so if they double vote, then the second vote will be rejected. Likewise if you try to vote by mail and then at the poll, you are flagged before you even try to vote.
Other states that do this well don't start counting mail in ballots until after polls closed. They know if someone voted in person, so their mail in ballot is rejected before being opened and verified.
You get text messages each step of the process too. “Your ballot has been mailed”/“your ballot has been delivered”/“your ballot has been received”/“your ballot has been counted - thanks for voting”.