title: Shopify: Is it possible to update the price of a product at checkout in the liquid template?
body: <p>If a buyer purchases an item from a Shopify store, we would like to customise the system such that the buyer can select another product from a specific collection in the same store for free with that purchase i.e. buy a premium dvd and receive another dvd for free (where the free dvd must be choosen from a specific collection). </p>
  
  <p>Any suggestions on how best to achieve this?</p>
  
  <p>Would it be possible to edit (hack) the theme's cart template to set the price to zero for the  second free item added to the cart if it is in the specific "free" collection?</p>