We Hold These Truths
The post office handles the Christmas rush. They'll be fine. These are letters that will easily fit through the automated system (unlike Christmas packages). Your real problem is there is no infrastructure for processing mail-in ballots at scale if the Congress can manage to get universal mail in passed as an option. It should definitely be an option, but at this stage you're already late to the game to put in the equipment and hire personnel to get them vetted before the November election. This needs to be done NOW if you're planning to do it.