I've just looked at the Immigration department's website. In the past one was required to mail one's paperwork 10 days before the due date. They've changed this to 15 days before.

They will only process it on the date that it is due, regardless when they receive it. Remember to include a self addressed envelope with a ten Bhat stamp, and keep the mailing receipt.

FWIW, no argument about BKK, but my recent first experience in Chiang Mai, mailed 15 days early, they counted it as received a week early, and assigned a new 90 days report date based upon the receipt date of my report. So, I figured next time maybe best for me to mail it just a week ahead of time....... Ah, but I do not want to be in violation of even the smallest regulation.!

15 days is the requirement on the national immigration website

http://www.immigration.go.th/

- it is not specific to Bangkok (which is a different website).

http://bangkok.immigration.go.th/intro1.html

Surely the website is saying that you can mail the documents up to 15 days in advance, not that you have to.

My reading is that they have extended the time window during which you can report, rather than creating an extra 'requirement'.

You used to be required to mail 10 days early - now it is 15 days early by my reading. They must have received application prior to due date by mail or you will be required to visit and pay fine.

Do not believe this statement leaves any doubt:

Your registered mail must be sent to the Immigration office at least 15 days before the due date of notification.

http://www.immigration.go.th/