Turns out there is a huge discussion about this at Transit at HKG - Security charge payable before boarding? .

A couple of issues here:

(1) There are two well-documented taxes which appear in the itemized list of taxes for a trip HKG-BKK-CNX in J:

These match the taxes you paid on your eticket.

The latter tax was added fairly recently -- some discussion at http://www.scmp.com/news/hong-kong/e...rge-fund-third .

But you have definitely paid both the departure tax and the (recently added) construction fee and the prices you have paid match the quoted prices for the airport.

(2) There is a third tax that I am really confused by. Supposedly there was, as of 2014, a passenger security charge "which is collected directly from each departing passenger at Hong Kong International Airport"? I don't remember doing that. https://www.hongkongairport.com/eng/...s/pr_1128.html I also don't know that I remember that charge being included in my ticket. And I don't see a 50 HKD security charge listed in any of the ITA fare quotes for ex-HKG trips I'm looking up now or the UA quotes for similar trips. Weird -- where has that charge been coming from before and what is up with it now?

Based on these threads it really really really seems like a manager at Thai Airways station in HKG has decided that there is a 45 HKD security tax which must preferentially be paid in cash and must be paid in person at the airport for all TG flights on award tickets. This does NOT seem to be the longstanding "departure tax" (120 HKD) nor the recent 2016 "airport construction tax" (amount varies based on destination, but for shorthaul J, 160 HKD). It seems to be some third tax altogether. I've never had someone ask me to pay a security tax at the airport when flying ex-HKG on revenue tix or award tix and I am wondering whether this is a real tax or whether the Apr 2014 "airport passenger security charge" documentation ( https://www.hongkongairport.com/eng/...s/pr_1128.html ) is up to date.

Edit: ok, read the thread in Transit at HKG - Security charge payable before boarding? really closely. There is a poorly documented third tax, a 45 HKD security surcharge, which is not separately itemized on receipts and normally not collected from passengers at all (it's just included in the "fare" price of any ex-HKG ticket). Per notes from seawolf in that thread, airlines have begun passing this fee on to passengers who are traveling on an award ticket -- notably CX as of 2015 and now, apparently, TG as of 2016. The tax is so poorly documented that if it did rise to 50 HKD some time between April 2014 and now we would have no way of knowing.

I think UA probably owes you an $8 refund here; this is an extremely poorly documented fee and it's a bad place for the TG/UA commercial relationship to break down. If TG is unhappy with how much partners are paying them for trips, this shouldn't show up as a surprise cash shakedown at the airport.