Add text string in front of subject on New Order mail on local pickup.

/*
 * Add text string in front of subject on New Order mail.
 */
function twstudio_change_admin_email_subject( $subject, $order ) {
    global $woocommerce;
    $blogname = wp_specialchars_decode(get_option('blogname'), ENT_QUOTES);
    $subject = sprintf( '[%s] Ny kundeordre (# %s) - %s', $blogname, $order->id, $order->order_date );

    $shipping_method = @array_shift($order->get_shipping_methods());
    if ( $shipping_method ) {
        $shipping_method_id = $shipping_method['method_id'];

        // local_pickup
        if ( strpos($shipping_method_id, 'local_pickup' ) !== false ) {
            $subject = sprintf( 'CUSTOM TEXT [%s] Ny kundeordre (%s) - %s', $blogname, $order->id, $order->get_date_created()->format ('d.m.Y') );
        }
    }

    return $subject;
}
add_filter('woocommerce_email_subject_new_order', 'twstudio_change_admin_email_subject', 1, 2);