Version 0.8c - released for $10. After 2 weeks it cost $5. As soon as version 0.9 is released it will cost $10, after 2 weeks it will be available for $5. That's why you won't find a $10 level right now, since 0.8c is already on sale for $5.
I'm not sure if itch works with a surcharge. I would like it to work as you said, but the itch team seems to have a different opinion. Most likely you need to make a one-time payment of $10.
