$inputCookie  = JFactory::getApplication()->input->cookie;
$value        = $inputCookie->get($name = 'myCookie', $defaultValue = null);
$cookieExists = ($value !== null);
$inputCookie->set($name = 'myCookie', $value = '123', $expire = 0);

$app = JFactory::getApplication();

// Get the cookie
$value = $app->input->cookie->get('myCookie', null);

// If there's no cookie value, manually set it
if ($value == null)
{
    $value = HOWEVER YOU GET THE CART DATA
}

// Set the cookie
$time = time() + 604800; // 1 week
$app->input->cookie->set('myCookie', $value, $time, $app->get('cookie_path', '/'), $app->get('cookie_domain'), $app->isSSLConnection());

JRequest::setVar($var, $value, 'cookie');
JRequest::getString($var, $default, 'cookie')