My oiffice accepted the statements I printed out from my Bangkok banking app. I did not get asked for a letter from the bank or for the statements to be stamped. They accepted that the statements were two days out of date. I did a transaction and updated my pass book on the day and the only thing I got from the bank was a photo copy of it for free. As I was changing from banked money to monthly payments I had to porivide my UK bank statements to verify source of transfers. The io took a long time checking money out and money in then said yes we give extension! My bank book also showed all the monthly transfers. As people have said, it could be very different at your office. Mine is Nakhon Phanom.
1900 baht at my local immigration office. Costs the same at all offices but some have slightly different requirements. I now get my extension based in monthly payments so no need to tie up
I changed from an agent to getting my extension myself and my local office did not check that my funds had been in the bank as required by my previous extension. Its worth your while trying to get your next extension yourself for 1900 baht but go as early as you can either 30 or 45 days according to your office. Find out exactly what their requirements are first. Good luck.