Just to update my own thread, I tried again today to book flights on Thai Smiles website. Still receiving error message about unauthenticated credit card and incorrect OTP.

Used Skype to phone their call center. Option for English, which connected me to a very good representative with great English language skill.

No phone booking fee or anything. Took about 10 minutes to book all the flights and now I am all set.

Up until collecting payment, I thought the Thai Smiles website was pretty good. Too bad it craps out at the end.

Be interesting to know if anyone is able to complete transaction using USA credit card...