I see others have asked about paying the remainder for beta since they preordered and I was wondering if you could let me know about this as well when itchio gets back to you? I am also interested in this. Thank you!
Itch.io has responded, and "topping up" previous purchase isn't something that is currently supported. Currently, the only way to do it would be to ask for a refund, and then re-purchase with a higher value. If you want to do this, send me an e-mail to email@example.com with your itch.io account e-mail address (I'll relay to itch.io regarding the refund).