OP - I can't quite figure out how it is that AA wanted a "change fee". My suspicion is that Expedia collected $750 for the DL ticket, but could not issue it and thus, booked but did not pay for the AA ticket. AA had the reservation but simply had not been paid for it.

You can certainly initiate a chargeback of the Expedia payment, but that only gets you $750 and leaves you out the balance of $500. You certainly don't have a basis to dispute the AA charge as they simply collected the AA fare.

You will likely need to sue Expedia in small claims court in order to get the balance of $500. It may be worth sending a letter to that effect, but my guess is that you are going to have to sue.

Water under the bridge for OP, but for others: why on earth book on Expedia when you can book direct on a carrier's website? FT is a tiny sample and there are hundreds of examples of people who have had horrible experiences.

Sure, when things go right, it is fine. But, when they go wrong, you are really stuck. Rarely are there much, if any savings. And when they are they don't justify the hassle.