The USSC gave NC the green light to accept ballots for six days after the election. “Oh, look here are 5,000 from the 1st precinct in Durham, and another 10,000 from Raleigh, and What? This entire county in NE NC...we just found them all laying in a ditch”. Plenty of time to manufacture a Dem win.
So even though they may be received through six days after, they still have to be postmarked no later than election day.