Lower shipping price for certain products?

I have been trying to use shipping classes to make it so certain products will be cheaper to ship.

Say my flat rate is 9.99, I have setup a shipping class called “Light Shipping” and want to charge only 2.99 for those items.

I have tried adding the following as the calculation but it doesn’t work:

-7.00 * [qty]

How can I have it so a product assigned the “Light Shipping” class lower the rate applied by the flat rate?