Cómo establecer valores predeterminados en el pago de WooCommerce

Acabo de crear una página con revisión de pedidos usando el siguiente código

<form name="checkout" method="post" class="checkout woocommerce-checkout" action="<?php echo esc_url( $get_checkout_url ); ?>" enctype="multipart/form-data">
<h3 id="order_review_heading"><?php _e( 'Your order', 'woocommerce' ); ?></h3>

    <?php do_action( 'woocommerce_checkout_before_order_review' ); ?>

    <div id="order_review" class="woocommerce-checkout-review-order">
        <?php do_action( 'woocommerce_checkout_order_review' ); ?>
    </div>

    <?php do_action( 'woocommerce_checkout_after_order_review' ); ?></div>
    </form>
    <?php do_action( 'woocommerce_after_checkout_form', $checkout ); ?>
      </div>

He establecido el valor predeterminado en function.php de la siguiente manera :

function overridefields($fields)
{
    global $wpdb;
    session_start();
    //print_r($_SESSION);
    //$fields['billing']['billing_delivery_date'] = array('label'=>'Delivery Date','Placeholder'=>'Date','required'=>'false','readonly'=>'true');
    $select_r = $wpdb->get_results("select * from `register` where `userid`='18'");
    //print_r($select_r);
    $fields['billing']['billing_first_name']['default']=$_SESSION['f_name'];
    $fields['billing']['billing_last_name']['default']=$_SESSION['l_name'];
    $fields['billing']['billing_address_1']['default']=$_SESSION['address'];
    $fields['billing']['billing_address_2']['default']='';
    $fields['billing']['billing_city']['default']=$_SESSION['city'];
    //$fields['billing']['billing_state']['default']=$select_r[0]->state;
    $fields['billing']['billing_email']['default']=$_SESSION['email'];
    $fields['billing']['billing_phone']['default']=$_SESSION['phone'];
    $fields['billing']['billing_postcode']['default']=$_SESSION['pincode'];
    //$fields['billing']['billing_delivery_date']['default']=$_SESSION['delivdate'];
    return $fields;
}
add_filter('woocommerce_checkout_fields','overridefields');

pero cuando hago clic en el botón de pago, aparece el error


Country is a required field.
First Name is a required field.
Last Name is a required field.
Address is a required field.
Town / City is a required field.
State / County is a required field.
Postcode / Zip is a required field.
Email Address is a required field.
Phone is a required field.


¿Cómo puedo establecer el valor predeterminado en la página de pago de WooCommerce?

Este código que encontré podría ser útil:

<?php
  add_filter( 'woocommerce_checkout_fields' , 'default_values_checkout_fields' );
  function default_values_checkout_fields( $fields ) {
    // You can use this for postcode, address, company, first name, last name and such. 
    $fields['billing']['billing_city']['default'] = 'SomeCity';
    $fields['shipping']['shipping_city']['default'] = 'SomeCity';
         return $fields;
  }
?>

Luego use CSS para ocultar las entradas.

Referencias

1. Los nombres de los campos pueden ser encontrado aquí.
2. Para la función principal woocommerce_form_field()
3. Referencia de GitHub

Usar default y custom_attributes en la definición del campo para aplicar atributos personalizados a la dirección de facturación/envío y a los campos de la cuenta:
https://github.com/woocommerce/woocommerce/blob/master/includes/wc-template-functions.php#L2510-L2514

add_filter('woocommerce_checkout_fields', 'my_woocommerce_checkout_fields');
add_filter('woocommerce_billing_fields', 'my_woocommerce_billing_fields');

function my_woocommerce_checkout_fields($fields) {
  $fields['billing'] = my_woocommerce_billing_fields($fields['billing']);
  return $fields;
}

function my_woocommerce_billing_fields($fields) {
  // Note: Default value is only used if the user account does
  // not have any value for the meta field yet. (Empty value is
  // also a value.) Ensure that the value is set correctly.
  $fields['billing_city']['default'] = 'SomeCity';

  // Disable the field in the form.
  // (Note: Does not prevent different values from being posted.)
  $fields['billing_city']['custom_attributes']['readonly'] = TRUE;
  $fields['billing_city']['custom_attributes']['disabled'] = TRUE;

  return $fields;
}