tButton.addEventListener('click', function() {
        callServerToRetrieveCheckoutDetails();
      });

    } else {

      checkoutButton.addEventListener('click', function() {
        // For demo purposes
        introPanel.style.display = 'none';
        legacyPanel.style.display = 'block';
      });

    }
 
    function callServerToRetrieveCheckoutDetails() {
      // Here we would call to the server with the chosen line items, to retrieve
      // a `Checkout` object with all of the prices and shippi