when I had our marriage certificate done in US I had to send it to the Department of state for authentication to receive our Thai marriage certificate in Thailand.
You can only do this from the states and it’s from the department of state and it’s an authentication of your marriage certificate. The turn around for me was 9 weeks.
also, if your required to get this letter you will also need to have is stamped at the Thai embassy in the US. Turn around time was about 9 weeks for everything. Hope this helps you save a headache 🙂
you may want to check and see if you will need a letter from the department of state to authenticate your marriage certificate since your planning to get married in the US. Personal experience and my wife is also Thai.