ExMo - I don't know about you, but AA RTW desk printed out and sellotaped to the inside of each ticket coupon set a copy of the Tariff information including taxes.
I did have a big problem last year with CX and an increse in tax ex-HKG leading to an hours wait at the city terminal whilst they re-caculated all the taxes to see if I had paid the higher or the lower rate of tax!