Due to needing it, I modified this..modification.. to store the relative tax rates along with the zip codes in the database, and automatically plug in the correct one in a customers order.

In the attached archive is the modified ot_zip_tax.php to go in the '/includes/modules/order_total' folder, as well as an SQL file that has the information for every zip code in California (I believe...).

So, replace the file, run the SQL statements, and you should be good.

-Joel

PS - I hope it was alright to modify this and share it, all copyright info was left intact. Just let me know if this isn't kosher.

PPS - I'm not really a PHP coder, but it worked fine in my store.