I do not do steam, unfortunately. (I might in the future, but there are no plans to do so right now.)
You do not need to pay anything to download the ZAGS demo. You can also pay on itch.io through stripe (instead of paypal) I think?
You can download the game on itch and then add it to steam locally. (You don't need the itch client to download games from itch, even the paid ones.)
(I'm with you on paypal... I don't really like them as a payment processor either.)