Every once in a while, we run into a technical question that the community has not yet answered.  

We have a client running the latest version of Magento, however, there is a bug in the system that prevents zip+4 tax codes from accurately being calculated.  As this is a major issue, we have found a patch for this.

In order to fix this Magento zip+4 issue with not calculating Tax, you will need FTP access and a text editor.

  1. Download this file: app/code/core/Mage/Tax/Model/Resource/Calculation.php
  2. Create the folders to: app/code/local/Mage/Tax/Model/Resource/ then upload Calculation.php
  3. Look for (around line 235) in Calculation.php:
    $postcode = $request->getPostcode();
  4. Replace that line with:
    $postcode = substr($request->getPostcode(),0,5);
  5. Save it, and your Magento zip+4 tax calculation issue will be gone!

Feel free to comment if you have any issues with this magento tax+4 (or tax4) calculation issue.

Please, dont attempt this without some experience with coding; we do not warranty expressed or implied with this solution. Keep in mind, this is a temporay fix; we cannot be held responsible for any damage that may result of this patch working on your site.

This tax+4 calculation patch will be safe from updates, but hopefully, in due time, the Magento team will fix this bug.