@Sarah ************
it’s not overstay. Overstay is when you stay in the country longer than the stamp you received when you arrived allows.
90 day reporting is done whilst in the country having stayed more than 90 consecutive days. You could be on a yearly extension, not have to leave the country at all, be late for your 90 day report (outside the window of opportunity you described) and get the ฿2000 fine.
It’s ฿2000 however late you are.
This is nothing whatsoever to do with overstaying