Your original itinerary ex‐CUN came to £156.10. After doing some calculations on ba.com I came to the conclusion that this was the correct amount (including YQ) based on the sums of two separate itineraries (£51 + £25.50 + £74.50 = £151). I noted the small difference between the amount I calculated and the amount you paid as fluctuation in exchange rates between now and when you made the booking so I had no concern to think that you had underpaid any amount. Normally, if your itinerary had originated in the US with a domestic connection, this would not include US immigration and customs arrival fees.

I suspect now that you have been charged the hefty £70.40 Mexico Transportation fee (XO) for your new departure out of MEX. Is this fee now only charged for departures ex‐MEX and not CUN on through international itineraries? I cannot confirm this. The other difference in tax was the Int'l Airport Departure tax (Mexico, XD) as I had already mentioned earlier being almost double that of CUN. You will be paying £50.40XD departing MEX but you have already paid the £28.40XD departing CUN.

Even so, ignoring fluctuation in exchange rates, with the difference in XD being £22 plus £70.40XO the total difference in taxes should amount to £92.40. Based on the calculations I have made I cannot determine the excess amount in taxes that you have now paid. It's a shame BA does not send a full breakdown of these fees in any e‐tickets. I would be inclined to ask for an e‐ticket receipt printout of your itinerary when you next pass through LHR. For a simple change of departure airport in Mexico, coupled with the change fee, that is quite a hefty amount to pay.