It's possible to gum up the works, if enough people send 1st class letters, particularly to major cities of battleground states:
Arizona, Florida, Georgia, Michigan, Minnesota, North Carolina, Pennsylvania, Wisconsin.
Ideas include letters to places that don't even actually exist (in the address is somewhat realistic, it will waste even more time). But also real addresses, like companies. Be nice. Tell them how much you appreciate their products. For lulz write them about the importance of mail-in voting. You can mail hundreds of letters per day. For stealth, randomize the return addresses (otherwise, it might be possible to pull the mail from the system).
BE POLITE
BE BORING
==BE POINTLESS=
eg:
Dear 711 on 12th Ave,
You have the most delicious produce I have ever tasted, I thank you kind sirs. Keep up the good work!
PS:
Mail voting is important. Voting by mail is the only way to avoid catching COVID when voting!!!