You Must Show Your Passport to Send Parcels at the Post Office
If you are planning on sending a parcel in Thailand, both within the country and abroad, you now need to take with you some form of identification such as a passport. Starting from today, Tuesday 2nd June 2015, Thailand Post will ask Thais for ID cards and foreigners for passports before they can send the parcel. Failure to show identification will result in denial of service. The reason is due to the ongoing suppression of the drug trade by the Thai government. Apparently some people have been using the postal service to distribute drugs.
I just sent a letter to the USA with only one piece of paper in it. Very slim envelope. I had to show my passport and they entered the information into the computer.
So parcels only. Letters stuffed with drugs perfectly fine to send from the mailboxes on the street.
In Thai, it said when you are sending things. This doesn’t include letters. But, I guess if your envelope looks a bit bulky then they will classify it as a parcel. Only one way to find out.