add_action( 'gform_after_submission_6', 'send_sms', 10, 2 );

function send_to_third_party( $entry, $form ) {

    // Account details
    $apiKey = urlencode('xxxxxxx');

    // Message details
    $numbers = rgar( $entry, '8' );  // WORKS FINE!
    $first_name = rgar( $entry, '24' ); // WORKS FINE!
    $last_name = rgar( $entry, '25' );  // WORKS FINE!
    $sale_price = rgar( $entry, '18' ); PRODUCT FIELD - NO VALUES
    $sub_total = rgar( $entry, '17' ); // PRODUCT FIELD - NO VALUES
    $message = rawurlencode('message');

    // Prepare data for POST request
    $data = array('apikey' => $apiKey, 'numbers' => $numbers, "sender" => $sender, "message" => $message);

    // Send the POST request with cURL
    $ch = curl_init('http://thirdparty.url');
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $response = curl_exec($ch);
    curl_close($ch);

    // Process your response here
    echo $response;

}