You can easily setup shipping charges as a percentage of the total sales amount.
In your OPP control panel please follow the steps below:
1. Click on "Set Prices and Options"
2. Click on "4. Shipping"
3. Select "Percentage Shipping"
4. Define the % rate you want below and Save changes. (if you want to charge 5% of the total amount for shipping simply write 5 in the % box)
Note: You can also easily define a flat shipping fee, or enable free studio pick-up option form the "Shipping" page.