you must register when you arrive go to Jomptien immigration they will then give you the paperwork for extension , there are more people on this group more qualified than me to give you more information, last time I came I did not register, so went to immigration to extend,asked in the first office on the left for info they said they would do it for 4000 baht, having seen the que I paid it, if you can do it online do it
If you are worried about online visa just book everything and come, you automatically get 30 days, then apply at immigration for a extra 30 days, job done 60 days