This is one of the weirder things that seems to have happened. Both sides, as usual, are playing their boogeyman. 25% voted by mail in 2016, and nothing was heard about it. I can't imagine the volume of mail then will even come close to what holiday volume is like, so the capacity is there.
https://en.wikipedia.org/wiki/Postal_voting_in_the_United_States#cite_note-35
I don't think the USPS has time to sort through ballots to figure out who voted for who.