I guess you have not heard of office postage meters that have a date you can adjust to whatever you want or run a bunch of empty envelopes thru early.

Pre canceled stamps are stamps that do not need to be cancelled by the machine at the processing center. These are stamps bought by bulk mailers who receive a discount for mass mailings. I suspect there are a lot of those around. 

I don't believe overseas absentee ballots require postage at all from military members. And absentee ballots can be mailed anytime even a month before the election. And there is nothing in NC law that allows extra time for slow mail, no reason to believe 3 days isn't enough. There we no delays in NC mail and if there were a judge could rule to keep it open based on actual problems in particular areas.

So why not wait a month or two just to be sure. Do we ever have to really close the election.

Election are locally run operations. If there are problem with long lines should it not be taken up with the locals election officials and elected representatives who are responsible for the incompetence. Everyone can vote early with practically no lines.