I did not know that 2016 brought in just 10,000 mail voters yet over 1 million have voted so far and counting. For the sake of clarity to avoid allegations of corruption from either side it would all be about the ratio of mail votes that would be counted in Republican held states vs Democratic held states. One side will definitely dispute the other therefore I see this going to court for judges to award the Presidency unless there is a landslide victory for one of them.
The voter turnout is going to be at record low levels for this election. Apart from the COVID 19 pandemic, there are opinion polls which claim that a large part of the American population is not supporting either of the two candidates. Some may go out and vote for third party candidates as a protest, but the vast majority will simply abstain from voting.
Even with the pandemic, they're expecting record setting voter turn out. Over 1 million people have already voted, at this point in 2016 that number was less than 10k.