Originally Posted by cebyam
I can download them to my 'Kindle for Mac' though. But not my actual Kindle, nor my desktop to transfer to the Kindle itself.
I've overread this passage the first time.
Obviously your purchase DID succeed, so this can't be the explanation.
When in your account in the browser (PC or Mac, not on your Kindle): Did you try the "deliver to" feature? Explicitly send to your Kindle? And what does "delivery status" for your Kindle say?