How can I obtain an embassy letter for a visa extension in Thailand?

I am a US citizen in Thailand on a visa exempt status which expires 3/30 the same day my return flight is scheduled to leave. Based on what transpired yesterday, my return flight will probably be canceled (my Cathay Pacific flight was previously canceled and I then rescheduled with JAL) so I will need to go to Jomtien to request an extension. My question is about a letter from one's embassy that I recall seeing mentioned by a member of this group. I would greatly appreciate more info on this letter. Is the sample Canadian one that was recently posted what I need? Is it essential that I have one when I go to Jomtien? Has anyone had success getting one from the US Embassy in BKK? Also, any advice that would assist me and others in my situation would be appreciated.