Troubles with woocommerce_ajax_get_customer_details filter

Hi, is there anybody good at woocommerce in administration? Could somebody help me out with the "woocommerce_ajax_get_customer_details" filter? I have asked a week ago but I've got no answer. I'm trying to make my plugins custom fields editable in admin, but I'm just not able to pass values, even if set. If you want to try the plugin, the dev version is here: https://github.com/vyskoczilova/kybernaut-ic-dic. The fields are "billing_ic" and "billing_dic". And even if I try something like this, the value is not passed into the field. What did I miss?

add_filter( 'woocommerce_ajax_get_customer_details', 'woolab_icdic_ajax_get_customer_details', 10, 3 );
function woolab_icdic_ajax_get_customer_details($data, $customer, $user_id ){

   $data['billing']['_billing_ic'] = '1';
    $data['billing']['_billing_dic'] = '2';
    $data['billing']['billing_ic'] = '10';
    $data['billing']['billing_dic'] = '20';
    $data['billing_ic'] = '3';
    $data['billing_dic'] = '4';
    $data['billing']['ic'] = '5';
    $data['billing']['dic'] = '6';
    $data['billing']['_ic'] = '7';
    $data['billing']['_dic'] = '8';
    return $data;
}