title: Custom price calculation in prestashop
body: <p>I'm working on a Prestashop 1.5.x website, where I need to add a custom price-calculation rule for a specific product.
  My goal is to add 10 dollars per order, but PS adds extra cost by product quantity, so if you order 20 products it ask you 200$ instead of 10...
  I need to override the calculating process in /classes/Product.php, with something like:</p>
  
  <pre><code>if (product_id = 44) { $price = $price + 10; }
  else { $price = $price }
  </code></pre>
  
  <p>Have you got any idea?</p>