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Viewing post in Knightly Passions [1.21 version] 18+ comments
It seems that itch.io still does not support upgrading your purchase. From two months ago:
"However, each payment is distinct, so it’s not possible right now to “add more” to an existing payment to unlock additional content. So, as you described, if they would like to unlock an additional tier they would have to buy it again at the total price, and request a refund on their first purchase."
https://itch.io/t/1489111/question-regarding-pay-what-you-want-price-and-upgradi...
Yes, it seems that you can refund both $5 purchases by contacting itch.io and purchase it once for $10 (or the other way around). It seems that in this case they mostly accept the refund. By the way this issue is completely itch.io's fault. The itch.io developers have agreed that it is an issue since 2016 but never actually bothered to solve it. This is an issue for every game that offers multiple payment tiers.