I'm wondering if some of the check-in staff are making a little extra money by being dishonest. On my last domestic flight, 50 baht extra was asked of me until I refused to pay on the grounds that I had recently bought the ticket after the increase had already gone into effect. If you purchased your ticket in late December, TG was already collecting the tax at that time. I'd be very careful about giving anyone any extra money for the departure tax unless you're sure it wasn't included in your ticket. If you think it was included in your ticket and they still ask for it, write down the name of the person demanding the money and get a receipt, then check up later to see if you were double charged or not. If so, report the person who scammed you out of paying the tax twice. I'm sure there's a lot of honest mistakes being made, but I wouldn't at all be surprised if there are a number of less-than-honest airport workers pocketing a very tidy sum every day.