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 can click 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;
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, or Price per unit (depending whether your rate is per entire order or per item ordered);
Shipping Amount: 19.00;
Country: United States;
Click Submit to save the Rule.
If you will ship Internationally at a different rate:
Shipping Rate Calculator: Flat, or Price per unit (depending whether your rate is per entire order or per item ordered);
Shipping Amount: 35.00;
Country: All Countries (i.e. Rest of the World);
Click Submit to save the Rule.
Click Back to Admin, and you're done!