Edit a template-override version of /includes/languages/english/checkout_process.php, making the changes below. That'll add the "Action required:" prefix to the order's subject if the order was paid via Zelle.

Code:
define('EMAIL_TEXT_SUBJECT', (isset($_SESSION['payment']) && $_SESSION['payment'] === 'zelle') ? 'Action required: Order Confirmation' : 'Order Confirmation');
//define('EMAIL_TEXT_SUBJECT', 'Order Confirmation');