Don't hire or buy any tour or transportation on "Badaka 99" travel agency (near Khaosan Road). We booked a full day trip with them and arranged to pay early in the morning, right before departure. Few minutes before starting the trip the manager, called Gilly, said that the employee from the day before had made a "mistake" and decided to charge us 1,000 bahts more. That almost doubled the original price! He refused to lower the price to what was already agreed before and by this weekend it was already too late to get another trip with another agency on the same day, so he cleverly and aggressively said that either we paid the ripoff or we could leave.
Don't ever go there.
They are not listed on TripAdvisor for a negative review as they deserve (Badaka99), but hopefully some travelers may find this warning if they research for the agency name online.