Voting by mail doesn’t always involve USPS, either. In Oregon, we have mailbox-like ballot boxes where we can drop our ballot. The ballots are collected by county election staff and counted in county. Mailing through USPS is also an option and certainly a more convenient option for some, just a tidbit of info for those concerned about mail fraud. I’m a Poll Clerk in California. We always have drop boxes for mail-in ballots, and take a bunch all the time. I’m signed up to always vote by mail, and part of my routine is to “cast” the first ballot of the day by putting it in the dropbox.
The issue goes up to the Supreme Court, which unlike the 2000 election does not decide the election in favor of the Republicans. However, it indicates again that the December 14 Electoral College deadline must be met; that the president’s national security powers legally authorize him to investigate potential foreign country intrusion into the national election; and if no Electoral College slate can be certified by any state by December 14, the Electoral College must meet anyway and cast its votes. The Electoral College meets, and without the electors from those four states being represented, neither Biden nor Trump has sufficient votes to get an Electoral College majority.