You can accomplish that by assigning a Cost of 5.50 to the Package Type that you will assign to products which will charge shipping.
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: 0.00
- - Package type: select Small Box for your free-shipping product, or Regular Box for your other tangible products.
- Capacity: 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 your country and enter your zipcode (if you're outside the US, just enter 00000);
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. Then click Back to Admin, and you're done!