/**
  * Returns the Checkout URL including a token for this transaction. 
  */
 public function get_checkout_url()
 {
     if (empty($this->token)) {
         $this->request_checkout_token();
     }
     // Include the token in the href if the default href is not overridden
     return PayPal_Digital_Goods_Configuration::checkout_url() . $this->token;
 }