title: magento checkout: change shipping price
body: <p>Scenario:
  Change shipping price using variable parsed by design during checkout.
  I need to change the final shipping price using some variables sent by phtml checkout.</p>
  
  <p>Eg.</p>
  
  <pre><code>Shipping method 1: cost 33$
  
  Shipping method 2: cost 23$
  
  Shipping method 3: cost 10$
       special packaging checkbox (y/n) + 10$ (set it as yes for this example)
       assicuration:
  </code></pre>
  
  <p>I've inserted manually this checkbox, just in design but I need to increment final price and add a simple label that include this on shipment</p>
  
  <p>total should be</p>
  
  <pre><code>shipment total                                    20$
  </code></pre>