I searched around but wasn't able to get a clear picture, so adding another WP topic. I'm going to be leaving the country and need to cancel my extension of stay (non-imm +WP. My employer is very cooperative and wants to help do whatever is most convenient for me, so I'd like to try to cancel my extension of stay the date of my departure.

I believe from what I've read that it's possible to submit a letter stating the last day and they will cancel everything effective that day. Is this correct? How far ahead of time can this be done? Can I go in a week before with a letter stating the date of my planned departure as my final date of employment and get my extension of stay stamped cancelled for that date and be able to leave on that future date without having to pay for an extension or overstay? Who writes the letter, me stating my resignation date or the employer stating my last day (in Thai?).

Thanks a lot for the help.

Unless you have a valid re-entry permit and plan on returning to the country you can just leave the country at an airport without doing the cancellation of your extension.

You could take a termination letter from your employer to immigration a few days before the date on the letter and immigration will post date the cancellation.