The Fee does not appear in my account

I think I have everything configured correctly, but when I execute the call to the service, I discount the FEE but I do not know where it is applied since in my account they are not. This is the PHP code, I'm missing something?

$preferences = array('items' => array( array('id' => $listing['listing_id'],
                                            'title' => $listing['title'],
                                            'description' => $listing['description'],
                                            'quantity'  => (int) $transaction['quantity'],
                                            'category_id' => $listing['category_id'],
                                            'unit_price' => (float) ($listing['price'] + $transaction['shipping_cost']),
                                            'currency_id' => $listing['currency_code'],
                                            'picture_url' => $listing['images'][0]['url_170x135'])),
                    'marketplace_fee'   => 0.6,
                    'order_id'          => $transaction['order_id'] ? $transaction['order_id'] : '',
                    'external_reference'=> $transaction['transaction_id_decoded'].(isset($transaction['receipt_id']) ? "-{$transaction['receipt_id']}" : ''),
                    'payer'             => array(   'name' => $this->CI->session->userdata('first_name'),
                                                    'surname' => $this->CI->session->userdata('last_name'),
                                                    'email' => $this->CI->session->userdata('email')),
                    'back_urls'         => array(   'success' => site_url(array($this->CI->lang->line('site_url_cart_process_payment'), 'success')),
                                                    'failure' => site_url(array($this->CI->lang->line('site_url_cart'))),
                                                    'pending' => site_url(array($this->CI->lang->line('site_url_cart_process_payment'), 'pending')),
                                                    'payment_methods' => array( 'excluded_payment_types' =>  array( array('id' => 'ticket'), array('id' => 'atm'))))
            );

log_message('debug', "create_checkout:".json_encode($preferences));

curl_setopt_array($this->ch, array( CURLOPT_RETURNTRANSFER  => TRUE,
                                    CURLOPT_HEADER          => FALSE,
                                    CURLOPT_CONNECTTIMEOUT  => 3,
                                    CURLOPT_CAINFO          => dirname(__FILE__)."/cacert.pem",
                                    CURLOPT_POST            => TRUE,
                                    CURLOPT_POSTFIELDS      => json_encode($preferences),
                                    CURLOPT_HTTPHEADER      => array("Accept: application/json", "Content-Type: application/json"),
                                    CURLOPT_URL             => "https://api.mercadolibre.com/checkout/preferences?access_token={$access_token}"));

$create_preference = json_decode(curl_exec($this->ch));