well - lets say it like this - if you do good - nobody will complain to you - you in Thailand - make sure you have the support from the Village Headmen - what is legal, illegal and done everyday are different stories ....
volunteering is work - paid or unpaid. you might consider check that with the organisation you wanna volunteer for - what they are able to handle for you.
you can do only online if you did 90 days before ... you need to report to CW Immigration by mail. Make sure to be on time. sent all docs as requested (same you go in person and copy of your passports) and an self addressed envelope with EMS stamps to them. I recommend you go once in person and after you do it online :-)