Presuming the shipping rates you described will cover all your products:
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: Regular Envelope
- Capacity: 6
Click Next until you can Submit to save settings;
Repeat this for all your soap bar products.
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;
Regular Envelope: Cost = 5.50;
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 (i.e., nothing will be added to the per-envelope Cost);
Country: United States;
Click Submit to save the Rule.
Add another Rule if you will ship Internationally at a different rate:
Shipping Rate Calculator: Price per unit weight;
Shipping Amount: 1.00;
Country: All Countries (i.e. the rest of your Shipping Destination countries);
Click Submit to save the Rule;
Click Back, so you can specify the Reg. Envelope Weight = the difference between US vs. Int'l shipping);
Click Next to save that Weight setting.
Click Back to Admin, and you're done!