/**
* Amazon Payments Advanced is available if the following conditions are met (on top of WC_Payment_Gateway::is_available)
* 1) Login App mode is enabled and we have an access token from Amazon
* 2) Login App mode is *not* enabled and we have an order reference id
*
* @return bool
*/
function is_available()
{
$login_app_enabled = 'yes' === $this->enable_login_app;
$standard_mode_ok = !$login_app_enabled && !empty($this->reference_id);
$login_app_mode_ok = $login_app_enabled && !empty($this->access_token);
return parent::is_available() && ($standard_mode_ok || $login_app_mode_ok);
}