In order to setup your checkout for state and sales tax you will first need to ensure that you have the two new columns in your orders table.
The idea is that you will use the eCart tax tab to create the tax rules. You will need to define the condition for which these tax rules are applied. For the state tax the idea is that the user selects the state from a select list, you can then use this user selected value for a state specific tax rule, or have a dynamic tax rule for the state.
A state specific tax rule is static, if the user selects a certain state then you will need to have a rule defined for that state that specifies the percentage of tax to charge.
In a dynamic rule you have the states and rates in a table, then when the user selects the state you will filter a recordset based on that value. You then store that rate in a session variable. When you configure the tax rule you will just check to see that your state tax rate session variable is set, and if it is then charge a percentage referencing this state rate session variable. This way you can have a single state tax rule for all the states.
When you are storing the order details you would use the bindings for your tax rules for their respective columns, then record the order tax using the regular eCart tax binding, this is a total of the tax rules applied.
As for adding the tax rates to the products this may enable you to control the federal tax as a per item calculation, then when you create the federal tax rule you can just use the total of your applied tax calculation as your federal tax amount for the order. If any part of this is not clear or you need any further info post back with some specifics and we will be glad to help.