I wanted to post for two reasons. First, as a heads up to others not to make the mistake I did. I had sufficient trip credit for a round trip from US to Mexico, but because I wasn't sure of the return date, I booked the outbound only (with trip credit), figuring I could apply the balance to a one-way return trip when my plans were clearer. That was wrong. Trip credit can only be applied for itineraries originating in certain countries, and it turns out Mexico is not on the list. (Honduras and El Salvador are... go figure.)
Second, I'm curious if anyone can explain why it's not feasible simply to allow trip credit to be used on any AA itinerary? Apparently they are adding countries piecemeal, and they haven't made it to Mexico yet. I don't understand why it's so hard?
It has to do with countries being on different payment and ticketing rails, as well as coordinating local offices.
It is definitely a PITA, as someone who basically flies only EU-USA and has had to cancel flights not in that direction. I wouldn’t expect substantial improvement for a while.