Hi,
To have it only on the product page, you can probably use this code:
$ctrl = JRequest::getVar('ctrl','');
$task = JRequest::getVar('task','');
if($ctrl == 'product' && $task == 'show'){
echo "<html>zzgl. <a onclick=\"return window.hikashop.openBox(this,null,false);\" href=\"/index.php/component/content/article?id=4&tmpl=component\" rel=\"{handler: 'iframe', size: {x: 460, y: 270}}\">Versandkosten</a></html>";
}
But having html tags inside another html tags is not a good solution, you should have iframe tags around you html ones I think.
In the echo you can potentially add a code to call the desired stylesheet.
<link rel=stylesheet href="
www.yourdomain.com/style.css
" type="text/css">