I think there needs to be a great deal of clarification on this very issue.
Are these ballots postmarked either before or on November 3? If yes, ok. Count them as they are valid.
If postmarked 11/4 or after - too bad. Falls into the "you had one job" category for a voter. If I can't walk into my local election commission on 11/4 (or after) and cast a ballot, you can't have yours postmarked that day or after. If your tax return is after April 15, it's late and you have to pay a penalty. In the case of absentee voting, your penalty is your vote is late and doesn't get included in the official results.
To answer your question:
NO ONE is accepting ballots post-marked after November 3, 2020. No state has in their election code/law to accept ballots post-marked after November 3, 2020. It is law that votes submitted can ONLY be accepted no later than Election Day, which in this case was November 3, 2020. When voting in person, that means voting before the polls close, pursuant to the respective state election codes (in SC it was 7pm, in other states it was 8pm).
With mail-in/absentee ballots, there is variances regarding them because these are not in-person voting, but votes that come in from other locations that need to be tallied and physically registered into the logs by election officials. They need to be removed from the envelopes, confirmed they are authentic as to identities of the voter and was properly filled out and signed. And THEN added to the logs (registered).
This involves time delays with mailing of the ballot, and the extra physical work of gathering the ballots, unsealing the ballots, checking for verifications, and THEN registered. It takes additional time. And if there are issues with how the ballots were delivered to the election offices (USPS), then there could be delays with all that "processing".
Just like everyone else knows about all the other issues we're dealing with as a nation (Covid-19, economy, racial and political divides), everyone knows about the issues with the USPS, and mail delivery delays. During the election, even after Tuesday, there are reports of people finding bins of undelivered mail including ballots, of persons trying to drive across the Canadian border with his trunk full of undelivered mail, of the federal courts mandating that the USPS stay up all night scanning their warehouses for undelivered ballots stashed away.
So ALL these ballots are ballots that are post-marked prior to or on November 3, 2020. And as you stated above, they are considered LEGAL votes by the courts, and per the Constitution.
But because Donald Trump made it his 4-year mission to denounce and cast doubt on the system of mail-in/absentee vote as being fraudulent, THIS election there is extreme focus and attention being placed on them. Who knows - perhaps in past elections there were regularly large numbers of mailed in ballots that were thrown aside by the USPS, votes by US Citizens that were thrown into the garbage, never counting in those elections.
But thanks to Trump, they will most definitely be counted in this election. Everyone say Thank You, to Mr. Trump......