I also contacted an American firm in Bangkok and got this response:
Thank you for reaching out to Baan Thai Immigration Solutions. We do process US passport applications. If you need assistance in completing DS-28, our company can definitely assist you. Our service fee is THB 10,000 + 7% VAT.
We look forward to being of assistance and thank you again for connecting with us.
Best Regards,
Martin David Juarez
Immigration Specialist
www.BTISolutions.co
208 Bangkok Building, 208 Wireless Road, Suite 1102, Lumphini Sub-district, Pathumwan District, Bangkok 10330
Basically the same price except for the 7 percent VAT. If I am not mistaken, the UK firm said they could do it for 5000 baht since I had already paid my Embassy fee. I'm going to double check this and be sure before I proceed. All things being equal, this could be better for someone who lives in Bangkok rather than Pattaya.