Fair comment Kim. I sometimes forget that each office seems to make up its own rules. Since I'm travelling this weekend I will definitely be trying that myself later in the year.
You have to do it 14-7 days before the due date. Your TM30 must be current.
You have to go in person if it's the first time after overseas travel (or just the first time);or a new passport (the 90 day system is not linked to immigration).
I can't find the article where the bigwigs had a shindig in Bkk to promote the online service as it would 'free up immigration staff' or something, but I did chuckle when they started forcing people back to immigration offices again. Thai logic.