You can't just return your ballot in any envelope, you'd have to have access to excess return envelopes which is a tall task. Also, I'm not sure what you mean

You can't just return your ballot in any envelope, you'd have to have access to excess return envelopes which is a tall task. Also, I'm not sure what you mean by adding marks invalidating the ballot. If you fill out a ballot incorrectly, the poll workers will notify you that there are issues with your ballot.