I'm not sure what you may have had set up before, but it would be easier and more reliable to do this:
Go to Seller Admin > Cart Shipping Settings and set up the following:
Shipping Origin: select your country and enter your zipcode (if you're outside the US, just enter any 5-digit number);
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;
Shipping Amount: 0.00;
Country: Select your country;
Click Submit to save the Rule.
If you will ship Internationally at a different rate:
Shipping Rate Calculator: Flat or Price per unit;
Shipping Amount: [enter your Int'l shipping fee here];
Country: All Countries (i.e. the rest of your Shipping Destination countries);
Click Submit to save the Rule.
Click Back to Admin, and you're done! Just make sure you enable Shipping/Buyer's Address in the settings of each product that should apply your Shipping Rules and trigger collection of the buyer's address during checkout.