You can apply tax to shipping in custom CalculateShipping method (please see Overview and Using Custom Providers sections in E-commerce guide for more details about how to implement custom provider), that is used every time the shipping value is required, and return shipping including tax from this method. If you want also to display the tax in shopping cart content/tax table you can use custom EvaluateShoppingCartContent method - you can store calculated tax into cart.ShoppingCartCustomData hashtable in CalculateShipping method and then you can acquire it in EvaluateShoppingCartContent method in order to to display it in tax table.