Bangkok bank has a branch on the ground floor at CW so I just went in, waited a little after taking a queue ticket and asked for the letter and last 12 months of statements. As I recall they charged either 200 or 300 THB for the service.
With regard to the agent original Non-O application my point was that if the rules state unequivocally that the 800,000 THB has to be in the account for the 3 months immediately after the granting of the extension then this condition could not possibly be met since the 800,000 THB was only in for 5 seconds even before the process had begun. In other words at the moment of application for the Non-O visa I did have any money in my account at all.
I think the point that you are making perhaps is the the agent is able to bypass the 800,000 THB rule?