There are three shipping groups in total: Light, Bulky and items that are not assigned to a shipping group.
When products assigned to the “Light” or “Bulky” shipping groups are in the cart with products that are not assigned to a shipping group the shipping rates should add together. However, when “Light” products are in the cart together with “Bulky” products, the “Light” products should ship for free.
A shopping cart price rule will need to be configured for this.
First, the package_id attribute will need to be configured for use with shopping cart price rules. Go to
Catalog > Attribute > Manage Attributes and edit the package_id attribute. In the “Properties” tab change “Use for Promo Rule Conditions” to “Yes” and save.
Promotions > Shopping Cart Price Rules create a new rule and fill in the rule information.
In the “Conditions” tab select “Attribute combination”. Within that rule select “Shipping Group” under the “Product Attribute” sub-heading. State that the Shipping Group is “Bulky”. This denotes that there must be a product from the “Bulky” shipping group in the cart for the shopping cart price rule to take effect.
In the “Actions” tab set “Free Shipping” to “For Matching Items Only”.
In the conditions section of the “Actions” tab select “Shipping Group” under the “Product Attribute” sub-heading and state that the shipping group is “Light”. This denotes that free shipping should be applied to only products assigned to the “Light” shipping group.
Don’t forget to set one or more free methods as per the free shipping promotion guide.