Hmmm... posting/editing seems to be going wonky.

I wanted to add: On Expedia, I often find the same flights (both AA- and US-operated) sold for the same price on both airlines' ticket stock. So, for example, if I'm flying LGA-MIA-MEX (as I am on my next flight), the same flights pop up with both an AA flight number and a US flight number for the same price. That should solve your immediate problem, no?

(I realize there are lots of people who argue never to book with a third-party TA. I personally have never had a problem and feel that as long as you're a savvy, proactive traveler who knows when to follow up - e.g., if you don't get an email confirming ticket number and itinerary, don't wait until you show up at the airport before asking questions - you should be fine.)