Buying Main Cabin Extra on flights booked through QF

[bmdog]

Hey folks, trying to buy a main cabin extra seat on an upcoming flight in the states (domestic leg of an international itinerary - I'm flying Qantas SYD-DFW then AA DFW-PHL). Booked through Qantas.com but the domestic leg is booked under the actual AA flight number, not a QF codeshare. I can access seat selection through my AAdvantage account, but it won't let me buy the seat - when I enter my card details and hit submit it just gives me "We were unable to process your product selection". The weird thing is that it lets me select a regular seat without a hitch, it's only if I want a paid seat that it gives me an error. What's happening here? Should I be calling QF or AA?

 

[juddles]

just to understand a bit better - is it bombing out once you enter all your credit card details? Are you putting in a postcode?

 

[bmdog]

My God, I just tried it without the postcode (which it pre-fills from Aadvantage details) and it worked! That was the only part of the payment details that I didn't experiment with changing. What on earth. Thanks for saving me an hour on hold!