Once you've created a shipping method, you can fine tune the calculation based on the weight of the order. Use the "+" button to add new ranges, and specify what ranges will apply to what calculations. Choose a type of calculation to perform for each weight range - a number (flat charge), formula (arbitrary calculation based on the weight), UPS lookup (internal or via their www site), "Multiply by" (simple rate), or select "Message" to generate an error message. If you specify a formula, you can include the string "@@TOTAL@@" to represent the total weight of the order. For example, "8 + @@TOTAL@@ + 2.50".