I tried last year on line and couldn’t and I’m not sending all my stuff to Madrid, no way. So I shall do what I did at the beginning of this year and get my 30 stamp and go to the immigration office for n Samui and extend for a another 30 days.
Don’t go too early, no more than 12 days before the stamp in your passport runs out. Make sure you have all your paperwork in order and go early, in Samui they opened at 08.30