To be fair, what you are asking is not a standard request, and I don't believe you will find any cart system that will support this sort of tax calculation out of the box.
To accomplish this is going to take 4 main steps:
1) Create a tax rate table
2) Create a recordset to lookup the tax rate
3) Add a Set Session value to store the tax rate
4) Set the tax calculation to use the session value.
1) I think if you can break the tax rates up by city instead of zip code it would definitely simplify things, but either way, I think this solution is going to call for a new database table to hold the tax rate - zip code or city relation:
taxID - Primary Key column - Auto Number
TaxRate - Number - holds the tax rate
zipCode - varchar - holds the zip code - could be state if you decide to go that way instead
You will need to enter all of the zip codes and the corresponding tax rates into this table.
2) Then on the confirm page, create a recordset to lookup the tax rate from the database for the zip code that was entered.
To create the recordset, click the plus button on the server behaviors panel.
Set the name of the recordset to "taxLookup"
select the database connection to use, select the taxRate table.
Leave the recordset window in simple view.
In the Filtering section, Select the zipCode column set the comparison to '='
set the variable type to Form Variable and enter 'postcode' for the variable name.
3) Once the recordset is created, add the set session value server behavior to the confirm page.
Click the plus button on the server behavior panel and select eCart -> General -> Set Session Value
For the trigger select "Recordset taxLookup is not empty"
set the name to "taxRate"
click the lightning bolt next to value and select the taxRate column from the taxLookup recordset.
4) Now to create the tax calculation.
In the eCart object, on the tax tab, click the plus button to create a new tax rule.
For the trigger select Session variable Exists:
if the Session Variable 'taxRate' exists
for the calculation use Based on carts taxable subtotal:
Taxable subtotal 'Times' '$_SESSION['taxRate']'