At every 90 day report I make in person (because on-line only works one third of the time) the Immigration Officer asked how much I paid the Visa Agent and that it would be much cheaper to 'do it with us'. You can take that two ways! The other day I legitimately renewed my extension. My Thai Mrs gave the I.O. 500 baht extra. I asked her why? She said that it was not asked for but she gave it 'in case we need help in the future'. Tipping officials is built into the Thai culture. In other Countries also of course.