Sold an $85 book on Amazon, forgot to "confirm shipment," didn't get paid for it?

[A Rose for the Dead]

I sold an $85 book a few weeks ago, but I just realized I never got paid for it. I know that's because I forgot to "confirm shipment." I have just learned that if you don't confirm it, Amazon won't pay you, your item will be re-listed, and the buyer will get it for free. That's pretty f*cking stupid.

Is there ANY way I can prove that I shipped it and get my money, whether it's through Amazon or the post office? I called Amazon twice, and both times they couldn't really do anything. But a transaction cannot just disappear like this. They are the ones who run the website, so there HAS to be a way they can check to see exactly who has ordered from me, which shipments I have confirmed or not, etc....right?