The post office (return address is downtown Washington, D.C.) sent out postcards, mentioned above. I got another one, this time at my post office box. It turns out that they AREN'T helpful because they are stupidly mailed to all 50 states and the voting systems are different in all of those states. And it has been pointed out on news programs that the timing suggested is too short for ballots to truly make it back in time if mailed.

Colorado's state attorney general got an injunction prohibiting the post office from mailing any more of those because they use mail in ballots for all citizens and the PO card is just going to confuse voters.