Shipping costs cannot easily be set on a product-by-product basis. However, you can get around this limitation by setting products' and shipping methods' package sizes as instructed in this article.
MyCashflow's free shipping campaign allows you to set up free shipping for all products in your online store, but you can't limit your campaign to certain products.
Setting free shipping based on the package size
To enable free shipping only for selected products, set products' and shipping methods' package sizes in the following way:
If you defined the shipping method and the products as described above, customers will automatically be offered free shipping when selecting a shipping method if their order contains only products with a package size that matches the package size defined for the free shipping method.
If the order includes other types of products, the free shipping method will not be displayed on the selection page at all. The disadvantage of this solution is that the order cannot contain any other products than the ones for which free shipping has been defined.
If you would like to temporarily disable the free shipping method, simply hide it from all store versions.
Once you've created the shipping method and set its shipping costs, you can set the desired products to use the shipping method.
Set the package size that you selected for the free shipping method for all relevant products.