I'll look around, maybe I missed something. I'll let you know if I find something!
As far as I can tell everything is setup right. You might need to reload the purchase page. There should be a button. If it's still not working let me know.
I sent a support request to itch.io but they haven't replied yet. If I don't get an answer this week I can send you a key.
Sent you an email. Sorry for all the trouble. Thank you for being so patient!