Entering a weight of 0 will tell the cart that nothing is being shipped, and no shipping charges will be charged unless something with a weight over 0 is also ordered. This works well for brochures and catalogs as well. Also you can offer a shipping choice of flat rate or no shipping in addition to the normal shipping costs. For instance, you may want to allow air shipment if the customer pays the cost, but offer free shipping if they let you choose the method. In that case go to the shipping options, and modify the “First Class” shipping charges to be flat rate or free. Then change the text associated with that option to say whatever you want.