Weight-based shipping rates
From Spiffy Stores Knowledge Base
Weight-based shipping rates use the total weight of the shopping cart to determine which shipping rate will apply in calculating the shipping costs.
Add a weight-based shipping rate
Add a weight-based shipping rate
Shipping rates can be created and managed from the "Preferences -> Shipping & handling" page of the Toolbox.
Select the destination country and click on the "Add a weight-based rate" link. If no shipping rates are defined already then you will see a message informing you that rates need to be defined. Click on the "add one" link.