title: Get the value of a custom checkout field in woocommerce 3
body: <p>In Woocommerce checkout, I am adding a custom checkout field and here is my code: </p>
  
  <pre><code>add_action( 'woocommerce_before_order_notes', 'shipping_add_select_checkout_field' );
  function shipping_add_select_checkout_field( WC_Checkout $checkout ) {
      $options = array_merge( [ '' =&gt; __( 'Nothing to select' ), ], city_zone() );
      woocommerce_form_field( 'billing_country_zone', array(
              'type'     =&gt; 'select',
              'class'    =&gt; array( 'form-row-wide', 'address-field', 'update_totals_on_change' ),
              'label'    =&gt; __( 'City zone' ),
              'required' =&gt; true,
              'options'  =&gt; $options
      ), WC()-&gt;customer-&gt;billing_country_zone );
  }
  </code></pre>
  
  <p>Now I am totally lost as I need to know what is <code>WC()-&gt;customer-&gt;billing_country_zone</code> for and how can I check it's value… </p>
  
  <p>Any help is really appreciated.</p>