The Embassies of major countries are not issuing any such letter this time - have announced they will not. So it is not a simple matter of getting an Embassy Letter
I had to pay 15,000 Baht a month ago to get a One Year Extension of Stay of my B Visa. My Assistant acting as an agent got 3,000 of that and Immigration took 12,000. Immigration can charge what they want to. Thus the reason immigration pushes for use of agents as they do. In the past I have been admonished by the I/O for not using an agent.