You can do that with Shipping Rules. 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 1-item Domestic shipping:
Shipping Rate Calculator: Flat
Shipping Amount: 3.00
Max. Items in Cart: 1
Country: United States;
Leave all other settings blank or 0.00;
Click Submit to save the Rule.
Next rule for 2-5 items:
Shipping Rate Calculator: Flat
Shipping Amount: 6.00
Min. Items in Cart: 2
Max. Items in Cart: 5
Country: United States;
Leave all other settings blank or 0.00;
Click Submit to save the Rule.
Next rule for 6-25 items:
Shipping Rate Calculator: Flat
Shipping Amount: 13.00
Min. Items in Cart: 6
Max. Items in Cart: 25
Country: United States;
Leave all other settings blank or 0.00;
Click Submit to save the Rule.
Next rule for 26-49 items:
Shipping Rate Calculator: Flat
Shipping Amount: 17.00
Min. Items in Cart: 26
Max. Items in Cart: 49
Country: United States;
Leave all other settings blank or 0.00;
Click Submit to save the Rule.
Next rule for 50+ items:
Shipping Rate Calculator: Flat
Shipping Amount: 0.00
Min. Items in Cart: 50
Country: United States;
Leave all other settings blank or 0.00;
Click Submit to save the Rule.
If you wish to ship internationally at different rates, repeat the steps above but select Country: All Countries (this covers the rest of your Shipping Destination countries that don't have a Rule of their own); you can also set up other country-specific rules (e.g. Canada) that would be exceptions to the All Countries rule.
Click Back to Admin, and you're done!
