title: PHP Fatal error: Class 'WC_Shipping_Method' not found in custom class addon
body: <p>I've created a shipping method addon to WooCommerce: <a href="http://pastebin.com/SCsbDnnn" rel="nofollow noreferrer">http://pastebin.com/SCsbDnnn</a></p>
  
  <p>However, it doesn't work and gives this output: PHP Fatal error: </p>
  
  <pre><code> Class 'WC_Shipping_Method' not found in class-wc-shipping-per-product.php, on line 44.
  </code></pre>
  
  <p>Why isn't this class found? I've even included two files to make sure, but no go:</p>
  
  <pre><code>include ('/woocommerce/woocommerce.php');
  
  include_once( 'classes/abstracts/abstract-wc-shipping-method.php' );
  </code></pre>
  
  <p>And I have checked this other question, but mine might be different, thats why I added a new one: <a href="https://stackoverflow.com/questions/14972184/woocommerce-extending-wc-shipping-method-error-class-wc-shipping-method-not-f">WooCommerce Extending WC_Shipping_Method error: Class &#39;WC_Shipping_Method&#39; not found</a></p>