I try to do that same but American embassy dose not provide affidavit at all social security office dose so Chiangmai immigration doesn't accept monthly income ,deal but they said Japanese can do , hahaha
I don't know but I think it's up to the airline you flew with and what kind of passport u have , I been in, out for many times they never ask at all , also I was standing on the line at immigration , at airport, nobody shows anything other than passport, is that mean everyone was so lucky ???