Do you know what tax rates apply to the various zip codes? This will be necessary in order to charge the correct tax rate.
The idea is that you would have a set of tax rules. The first condition in the trigger for the tax rule would be that the state selected was WI. The next conditions will be if the entered zip matches a particular set of zip codes. If so then the tax rate for that set of zip codes would be used.
You would then have a different tax rule constructed like this for each potential tax rate.
If you have the info in a table in your db that holds the zip and corresponding tax rate you could do a recordset lookup of the tax rate based on the user submission and store the tax rate in a session variable. You could then have a rule that is triggered on the presence of this session variable, then use the session variable again in the calculation for the rate.
The first path is the static one, the second is the dynamic way to go. Let us know what direction you would like to take and include any questions that you have about the specifics.
