It would help to know the details surrounding the dispute.

But based on the information provided, I'm surprised to hear that AMEX is giving you such a hard time. I've never had to file a dispute, but have used the purchase and return protection benefits extensively for purchases varying between $8-$500 and have always had my case resolved in my favor within 24 hours. They didn't even make me go through the hassle of shipping the item in question back to them.

With that said, have you tried a return protection claim?