Currently, this functionality is only available if you are using Real-Time Shipping. At this time, you can not limit Custom Shipping based on destination. However, you can set the Allow International Shipping Address to be NO under Checkout >> Shipping > Shipping Manager. Once that is set to No, and your stores shipping origin is set to a US address, you will see that within the first stage of checkout the address must be within the US to go to the next step.
