To close the loop on this in case it is of some use to anyone else in the future :
Following the call I mentioned here earlier where AA reservations told me it was a domestic flight from Miami to Cancun(!) and that I could just sort it out at the airport I received the email below today ... I guess this was related to the Infant that was added and not ticketed on the booking as the booking for my wife and I was ticketed and paid for already.
Please call us to talk to our Customer Service teams. We are unable to process your booking. Booking reference xxxxx. Click here to contact us.
We look forward to speaking to you.
I called AA reservations on the UK 0207 Number and an extremely helpful guy named Paul sorted it all out. He sounded like he was UK based, I guess during the week the UK number is answered locally and at the weekend it goes elsewhere.
Anyway he told me to ignore the email, he understood that MIA-CUN is not a domestic sector and he got it ticketed for me with the help of his rates desk, just took a couple of minutes on hold..
I now have an eticket for the infant on the booking so I'm happy that we won't have problems en route.The final cost was £11.70 ( so a little under $20 ), e-ticket shows it as all taxes and the base fare as 0.00
I guess it probably would have worked out the other way too, but I suspect it would have been more hassle and stress so it seems easier to get it sorted this way ... Still seems strange how difficult it is to book infant tickets with AA, they really don't make it that simple !