That is great that you have a table with the info. So the idea is that you will create a recordset on the confirm page. This recordset will be filtered on the posted zip value if the selected state is WI.
To make sure this recordset is only created when the user selects WI you will need to add a custom if statement around the rs, it might look like this depending on your checkout form name for that field:
if(isset($_POST['state_province']) && $_POST['state_province'] == "WI"){
// the recordset goes here
}
Once the recordset is crafted you will add in a set session value server behavior. You give your session variable a name and for the value use the bindings to store the tax rate in this new variable.
Once you have done that you can craft the tax rule for the zip code. Create a new tax rule and for the trigger you will have two conditions. The first condition will be that the session variable exists and the second condition will be that it is not "" or blank.
Then for the calculation portion you will use the subtotal for the entire cart and multiply it by your session variable. You will need to enter your session variable like you would in code:
$_SESSION['tax_rate']
Please post back with any questions that you may have about any part of this.