The Magento/Piwik Community now has a free and open source plugin to simply setup Ecommerce tracking with Piwik in your Magento shop!
Created by Adrian Speyer, this new plugin works on the community edition of Magneto (1.7) and is loosely based on the Google Analytics core extension. Once you install it, you will find it in the Magento menu under Sales:
The new plugin is quick to install, all you need to do is enter you site id and your Piwik path. If the configuration page does not show up, make sure you refresh your cache.
Once enabled the plugin will automatically insert the Piwik Tracking code into your Magento install. Not only will it give you the standard tracking, it automatically sets up the Ecommerce tracking that you would expect: product & category views, cart updates, orders and more.
You can download the Magento extension here. If you want to fork or view source code, it is available on Github. He has also provided a quick YouTube video with a rundown of how it works and how to use it. There is also a Youtube Video tutorial here.
You can also check out Adrian’s other free Piwik addon’s for Concrete5 and Vanilla Forums. If you need more detailed instructions or have questions, contact Adrian via his blog post on the extension: http://www.statstory.com/piwik-ecommerce-extension-for-magento/
PS: Check out the other Ecommerce plugins for Pretashop, osCommerce and Oxid esales on the Plugins page.