we are in the process of renewing my passport now. Following the instruction from the embassy web site is not accurate. There is NO courier pass back envelope per se that you can buy from any post office. I went to 2 post offices and they did not know anything about it. You have to buy it from the Thailand Post online using the link provided in the instruction (mailing the passport section). Once you paid the 100 Baht, at the bottom of the page, the purchase invoice number will appear (starting with THP...).
You then write this number in the "check list" page that must be included in the package that would contain all the check listed items and send by courier to the embassy. Courier must be either DHL or Thailand Post only.
I hope this helps. I wasted two trips to the post offices plus one more trip to mail the package.
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