I have a similar question to the other two asked. A year ago I purchased the preorder for $30. Is there any way I can pay $19.99 to have access to the Toru beta without paying all over again ($49.99)? When I clicked buy and input $19.99 it wouldn't let me and said I had to make a minimum purchase of $28.99 to buy the full game (that file is already unlocked for me) and $49.99 to have access to the Toru beta (that file was grayed out unless I entered $49.99).
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 meyaoigames@gmail.com with your itch.io account e-mail address (I'll relay to itch.io regarding the refund).