title: Retrieve product custom field values in checkout custom fields
body: <p>I have a form in a single product page where user insert his informations (name, phone, country ,age ...) what I need to do is to display this information in billing details . </p>
  
  <p>So, what I did is I added 2 fields to my form in single product , here is the code : </p>
  
  <pre><code>&lt;div class="col-md-6"&gt;
                  &lt;div class="form-group"&gt;
                      &lt;input class="form-control alt" name="billing_vol" type="text"
                             placeholder="&lt;?php esc_html_e( 'N° Vol : *', 'rentcar' ); ?&gt;" &gt;
                  &lt;/div&gt;
              &lt;/div&gt;
              &lt;div class="form-group"&gt;
                  &lt;input class="form-control alt" type="text" name="billing_cli_age"
                         placeholder="&lt;?php esc_html_e( 'Age *', 'rentcar' ); ?&gt;"
                       "
                  &gt;
              &lt;/div&gt;&lt;div class="col-md-6"&gt;
                  &lt;div class="form-group"&gt;
                      &lt;?php 
                          global $woocommerce;
                          $countries_obj   = new WC_Countries();
                          $countries   = $countries_obj-&gt;__get('countries');
                      ?&gt;
  
                      &lt;?php 
                          woocommerce_form_field('billing_country', array(
                          'type'       =&gt; 'select',
                          'class'      =&gt; array( 'chzn-drop' ),
                          'placeholder'    =&gt; __('Country'),
                          'options'    =&gt; $countries
                          )
                          );
                      ?&gt;
  
                  &lt;/div&gt;
              &lt;/div&gt;
  </code></pre>
  
  <p>Then I did the same thing In checkout page in  billing detail Form, I have added this 2 fields. Here is the code : </p>
  
  <pre><code>add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
  
  function custom_override_checkout_fields( $fields ) {
  
      $fields['billing']['billing_cli_age'] = array(
          'label'         =&gt; __('Age ', 'woocommerce'),
          'placeholder'   =&gt; _x('Age ', 'placeholder', 'woocommerce'),
          'required'      =&gt; false,
          'class'         =&gt; array('form-row-wide'),
          'clear'         =&gt; true
      );
  
      $fields['billing']['billing_vol'] = array(
          'label'         =&gt; __('N° vol', 'woocommerce'),
          'placeholder'   =&gt; _x('N° vol', 'placeholder', 'woocommerce'),
          'required'      =&gt; false,
          'class'         =&gt; array('form-row-wide'),
          'clear'         =&gt; true
      );
      return $fields;
  }
  </code></pre>
  
  <p>Now I'm stuck , I don't know how to send the information added by user in form (that I have in single product) to billing details form in checkout Page.</p>
  
  <p>How can I achieve this?</p>
  
  <p>Thanks. </p>