add_action('woocommerce_after_order_notes_add_nip', 'socialking_add_nip_to_checkout');
function socialking_add_nip_to_checkout($checkout)
{

    echo '<div id="wpdesk_vat_field"><h2>' . __('Dane do Faktury') . '</h2>';

    woocommerce_form_field('vat_number', array(
        'type'          => 'text',
        'required'      => false,
        'class'         => array('vat-number-field'),
        'label'         => __('NIP'),
        'placeholder'   => __('Wpisz NIP, aby otrzymać fakturę'),
    ), $checkout->get_value('vat_number'));

    echo '</div>';
}