Okay, I don't know how new this information is, as I've been regularly checking the American Embassy page for how I could get another letter so that I could extend my visa, but from what was posted today, if I'm reading this message correctly, you can no longer get letters from the American embassy, but can get a 60 day visa directly from immigration (for a fee that they haven't specified on the website). Here is the info I screenshot from the U.S. Embassy in Thailand dated today: