A lot of misinformation above.
Letter mail to the United States is accepted as long as the letter does not contain merchandise. Registration is available but not required. My local Hua Hin 77111 post office routinely accepts letter mail to the United States.
The rate depends on the weight of the letter and the service requested. These are posted and updated on the Thai Post website:
https://www.thailandpost.co.th/?page=rate_result_nrs3&to_CountryID=210::United States of America&parcel_weight=