If I'm understanding correctly, you offer Free Shipping to the US, but you want to set up USPS Priority Mail rate lookups for destinations outside the US, and tack on an extra % for Handling to non-US buyers? If so, here's how you'd set that up:
Go to Seller Admin > View/Edit/Delete Products:
Select an existing shippable product of yours and click Edit;
On the first screen, if Shipping/Buyer's Address is not checked there, tick that box;
Click Next until you reach the Shipping Options screen;
- Weight: [enter an accurate per-unit weight for the product]
- Package type & Capacity: [if you will ship a single parcel for each entire order, select the same Package for all products and set their Capacity to 9999]
Click Next until you can Submit to save settings;
Repeat this for all your tangible goods;
When adding new shippable products, remember to enable Shipping/Buyer's Address.
Go to Seller Admin > Cart Shipping Settings and set up the following:
Shipping Origin: select United States and enter your zipcode;
Shipping Destinations: if you will only ship to certain countries, select them in the list, or otherwise select nothing for Worldwide shipping;
Handling: Select "% of Shipping" and enter the percentage of Priority Mail rates that should be tacked onto non-US orders;
Leave all other settings blank or 0.00;
Click Next to save settings and proceed to the Shipping Rules screen.
In the Existing Shipping Rules menu, select and Delete any Rules you may have saved there.
Set up your first Shipping Rule for Domestic shipping:
Shipping Rate Calculator: Flat
Shipping Amount: 0.00;
Country: United States;
Click Submit to save the Rule.
If you will ship Internationally at a different rate:
Shipping Rate Calculator: USPS Priority;
Country: All Countries (this covers the rest of your Shipping Destination countries that don't have a Rule of their own);
Click Submit to save the Rule.
Click Back to Admin, and you're done!