title: save woocommerce billing custom field in database
body: <p>I added one custom field in woocommerce billing fields..please check this bellow code</p>
  
  <pre><code>add_filter( 'woocommerce_billing_fields', 'custom_woocommerce_billing_fields' );
  
  function custom_woocommerce_billing_fields( $fields ) {
     $fields['duplicate_billing_address'] = array(
  
          'type'          =&gt; 'checkbox',
          'label'     =&gt; __('Duplicate Billing Address to Shipping Address', 'woocommerce'),
          'required'  =&gt; false,
          'class'     =&gt; array('form-row-wide'),
          'clear'     =&gt; true
       );
  
      return $fields;
  }
  </code></pre>
  
  <p>How to save this custom field value in database.</p>