For the first time ever, I had this happen to me too. I spent forever trying to checkout, had to keep logging back in, and my temporary cart kept disappearing. Over and over.
I finally started by logging in
Then added something else to my cart, then the Phantom one.
Next I went to my temp cart and moved them to my wish list.
In the wish list cart, I removed the other one I had added.
Finally, I was able to get the Phantom one.
BUT
The site won't let me stay logged in to get the download. This has never happened before at Tantor for me and the customer service is closed for the weekend.