Have a look at this on the Immigration web site.
Report 90 Day
Attention: Simplified Procedure for 90-Day Registration by Mail
You may now renew your 90-day registration by mail
Procedure:
Send registered mail to:
90 Day Registration
Room 311, Immigration Bureau
Soi Suan Pluu
South Sathorn Road
Bangkok 10210
Enclose:
- 1 photocopy of the photo page of your passport.
- 1 photocopy of the page containing your most recent entry visa.
- The original of your most recent 90-day registration receipt.
- The original of your new 90-day; registration receipt. Don't forget to sign it!
- A stamped, self-addressed envelope to receive your new receipt.
: Please allows at least 1 week for delivery of registered mail.
Your new form will be stamped as of the expiration date of your old form.
Important:
- If you have passed the 90-day limit, you must come to Room 311, Immigration Bureau in person.
- To receive a blank registration form, send a stamped, self-addressed envelop to the address above. Request form "TM 47."
Source:
http://www.immigration.go.th/forum/index.php?topic=122.0
It seemed that it used to be a little easier to go to AEK and do the report and than to make the copes and send the packet by registered mail, but now that it would mean a drive to Nongkhai, I think mail-in may be in order.