Adding Percentage Based Charge To Shipping

Hello,



I'm wondering if anyone knows of a way to a percentage based charge to a shipment that's been calculated using a live service?



For example. I've set up shipments to the US to connect to Fedex to give us live shipment charges, but I wish to add a 10% charge to the shipment costs. So if a customer is charged $30 for shipping, I'd like to add $3 to the shipping charges.



I know Shipping freight gives us an option to charge additional shipping costs, but I'd like to add the charge based on the actual shipping amount and also only to shipments going from Canada to the US.



Thank you for any help or advice.