Per section 7 here — https://www.mastercard.us/content/dam/public/mastercardcom/n... — MasterCard could simply remove Australia from Itch’s Area of Use, at which point they would not be permitted to accepted MasterCard from Australian customers, which the merchant could trivially enforce by country filter on the billing address.

I suspect we’re going to find out that Stripe is unwilling to risk losing Mastercard in Australia and also unwilling to implement passthrough AoU restrictions to their sublicensees, and Mastercard isn’t willing to act against any single customer of Stripe or else they don’t profit from the “not our problem” discount rate they issue Stripe to make it their problem.