He can still seek legal advice. Just because a contract says "we can't be held responsible for anything" doesn't mean it's always going to hold up in court. It just wouldn't be fair, and lawyers would lose too much business.
What matters is the disputed amount ($13,000), whether the author wants to play hardball, and whether Amazon would rather pay him to go away or squash him like a bug (to set an example). Because even if he wins (and I've no idea if he can), it wouldn't be fun if Amazon got nasty.
I'm not a lawyer, and I haven't seen the contract in question, so I have no real idea, but it's not as clear cut as "you agreed not to sue us". It could very well be a bluff by Amazon.
The theoretical - yes, you're right. It may be that the contract isn't airtight, though given that this appears to have been a genuine mistake rather than malice or profiteering on Amazon's part my suspicion is that the damage waiver isn't that unreasonable.
The practical - $13,000 is nothing in the world of legal fees, no-one in their right mind would sue a multi-billion dollar company over such an amount, especially with a case that's not clear cut and that's the absolute maximum financial loss you can assign to it and there's a reasonable argument that the figure would be lower.
Personally I think that Amazon should come to a goodwill settlement of a few thousand dollars with him but I suspect that's why I'm the one who hands over silly amounts of money to Jeff Bezos each month rather than the other way around.
What matters is the disputed amount ($13,000), whether the author wants to play hardball, and whether Amazon would rather pay him to go away or squash him like a bug (to set an example). Because even if he wins (and I've no idea if he can), it wouldn't be fun if Amazon got nasty.
I'm not a lawyer, and I haven't seen the contract in question, so I have no real idea, but it's not as clear cut as "you agreed not to sue us". It could very well be a bluff by Amazon.