To apply one Tax rate for all your products, you can use Flat tax rate option. Simply go to the Tax menu, click the Flat Rate radio button from Tax Settings, and enter the tax rate in the empty field.
There are two logics for Flat Tax rate. By default, Flat Tax Rate will be only applied to the products that have Apply Tax selected “Yes” under the product. Flat tax rate will override all the Product Specific Tax rates. Flat Tax will not be applied to the products that have Apply Tax selected “No.”
Second logic allows you to apply Flat Tax rate on all the products regardless if they have Apply Tax selected “No.” Flat Tax rate will always override Product Specific Tax rates.