fixed missign config file and added check if xmlrpc istnt installed

modules/billing/payment_success.php

@@ -5,7 +5,7 @@
  * Standalone billing module - uses only standard PHP mysqli
  */
 session_start();
-require_once(__DIR__ . '/includes/config.inc.php');
+require_once(__DIR__ . '/includes/config_loader.php');
 
 // Variables from config.inc.php (helps IDEs understand scope)
 /** @var string $db_host Database host */
@@ -223,4 +223,4 @@ if ($db && $user_id > 0) {
         </div>
     </div>
 </body>
-</html>
+</html>