Ok. So even though the order details page shows the total at $100, I can confirm it and then add in additional shipping costs afterward?
While you are confirming it, there is a field to add in shipping costs. But you can also geekmail the user directly. The field is optional -the commission is only calculated on the game price. I often geekmail a buyer with shipping options (different box sizes, or First Class versus priority), and let them choose, then I fill out the field when I confirm it and the details have been settled.