You can set up tax and shipping rules for the eCart Object by opening the eCart Object floater and editing the eCart Object that you want. There is a section for adding tax rules and a section for adding shipping rules.
Also when you are running the Checkout wizard, you can specify the shipping provider and you will be prompted for the shipping provider info. After specifying the shipping information you will be able to set the tax information.
You are not adding the shipping or tax information to the add to cart button.