And after first notification you will get an information about the next one, then you can do by registered mail or online (website/application). If you leave country, counter resets to 0 since the day you get back. If you do another extension of stay, counter DO NOT resets, still the date from previous notification of stay is in order.
With first extension you should get information to do 90 days notification. You count it since the date on the stamp (this date is the first day). First one must be done in person. The notification period window is from 15 days before due date, till 7 days after due date.
90 days notification is different than TM.30. 90 days is to be done on TM.47 form. TM.30 is only when you change address and you must do it within 24 hours after such event.
Last time I sent it 13 days before the due date and got it back after one month with annotation "Next time, please send 15 days before the due date".
Remember to include envelope with your address and 10 THB stamp glued to it.
I didn't send TM.30 receipt, and they didn't mention anything about it (however, they might want it if you changed the address since last 90 days notification). Also, they returned all photocopies of passport and TM.6 card.