We've gotten a few requests for a feature like this, but I doubt it'll be ready in time for next Friday. That said-- you can manually change what percentage of each sale goes to itch.io from your dashboard and then change it back at the end of the day.
Yes it will. The revenue share value is stored at the time of the sale, so changing it later will not affect any previously made purchases, even if you're getting paid for them later.
