Hi,
The issue is that if you add the display of a related products module in the popup, the module won't know for which product you're calling it. In fact, it doesn't even see itself on a product page and so it won't display the related products of the product you added to the cart.
Following the code of the other thread, you should also add such code:
$_REQUEST['product_id']=$product->product_id;
$_REQUEST['task']='show';
after the line:
$class = hikashop_get('class.currency');
echo $class->format($product->prices[0]->price_value,$product->prices[0]->price_currency_id);
That should help the module to display itself after the display of the information of the product added to the cart.