Hi there! I think there’s a way to do it by emailing itch.io support with your purchase ID at support@itch.io. You can access their general support page here at https://itch.io/support.
I believe refunds should be issued within 30 days of contacting them, but if they ghost you or that doesn’t happen by then, I’m happy to Venmo or PayPal you the $20 myself directly!