If you’re in Bangkok, I highly recommend this company. I’ve used them over 10 times.
Meet at Lotus Sukhumvit 50 at 4am. Leave at 4:30. They stop half way to use the washroom and to grab something to eat. They do that both ways. Once at the border, when you’re finish you get a free meal. Back to Bangkok between 2-3pm.
Cost 3500฿ for westerner. Check their webpage for all the info you’d need.