Free shipping with explanatory text, how to do?

I would like to put explanatory text after the free shipping field on my website.

When the customer sees the option free shipping ($ 0.00), below it he wanted to put: "Sending will be done via PAC."

I noticed that the file you edit is the file free.php located in: \catalog\model\extension\shipping - even after some tests I could not insert. How should I proceed?

free.php

<?php
class ModelExtensionShippingFree extends Model {
    function getQuote($address) {
        $this->load->language('extension/shipping/free');

        $query = $this->db->query("SELECT * FROM " . DB_PREFIX . "zone_to_geo_zone WHERE geo_zone_id = '" . (int)$this->config->get('free_geo_zone_id') . "' AND country_id = '" . (int)$address['country_id'] . "' AND (zone_id = '" . (int)$address['zone_id'] . "' OR zone_id = '0')");

        if (!$this->config->get('free_geo_zone_id')) {
            $status = true;
        } elseif ($query->num_rows) {
            $status = true;
        } else {
            $status = false;
        }

        if ($this->cart->getSubTotal() < $this->config->get('free_total')) {
            $status = false;
        }

        $method_data = array();

        if ($status) {
            $quote_data = array();

            $quote_data['free'] = array(
                'code'         => 'free.free',
                'title'        => $this->language->get('text_description'),
                'cost'         => 0.00,
                'tax_class_id' => 0,
                'text'         => $this->currency->format(0.00, $this->session->data['currency']),
                'texto_informacoes'        => $this->language->get('texto_informacoes') 
            );

            $method_data = array(
                'code'       => 'free',
                'title'      => $this->language->get('text_title'),
                'quote'      => $quote_data,
                'sort_order' => $this->config->get('free_sort_order'),
                'error'      => false
            );
        }

        return $method_data;
    }
}