So, I looked at the trustedshops thing on the internet and other shopping carts always add it at the end of the order, on the thank you page.
So, it will depend on the payment method you use.
If you use the "collect on delivery", the "bank transfer" or the "check" payment plugins you will want to edit the file end.php of the view checkout.
If you use the external payment plugins like paypal, you will want to display it when the user returns on your website and so you will want to edit the file after_end.php of the view checkout.
There you will have to load the order information like this:
$app =& JFactory::getApplication();
$order_id = $app->getUserState( HIKASHOP_COMPONENT.'.order_id');
$orderClass = hikashop::get('class.order');
$order = $orderClass->loadFullOrder($order_id);
You will have all the information you need about the order in the $order variable.