Shopify.updateCartAttribute

// ---------------------------------------------------------
// POST to cart/update.js returns the cart in JSON.
// To clear a particular attribute, set its value to an empty string.
// Receives attributes as a hash or array. Look at comments below.
// ---------------------------------------------------------
Shopify.updateCartAttributes = function(data, callback) {
  var params = {
    type: 'POST',
    url: '/cart/update.js',
    data: data,
    dataType: 'json',
    success: function(cart) {
      if ((typeof callback) === 'function') {
        callback(cart);
      }
      else {
        Shopify.onCartUpdate(cart);
      }
    },
    error: function(XMLHttpRequest, textStatus) {
      Shopify.onError(XMLHttpRequest, textStatus);
    }
  };
  jQuery.ajax(params);
};