If you have a specific product that you would like to have a fixed fee for and adjust the shipping cost not to take that product into account accordingly, then you need the code from this repository. There are two seperate functions involved:
function wc_ninja_manipulate_shipping
which takes care of adjusting the shipping cost not to take into account a particular vendor products or productfunction woocommerce_custom_surcharge
that adds $30 surcharge fee below the shipping methods on its own line. Keep in mind: I am using the Table Rate Shipping plugin as well as my shipping costs are based upon order total for this example.