Hi,
It's that code which displays the popup when the product page refreshes after the add to cart:
if($app->getUserState( HIKASHOP_COMPONENT.'.popup', '0')) {
$js = $this->getJS();
$app->setUserState( HIKASHOP_COMPONENT.'.popup', '0');
}
in the file components/com_hikashop/views/product/view.html.php
So you would have to change the content of that if and instead do a redirection ( the header function of PHP on the location ).
Doing it for only one product is dead simple. Just check on the cid in the $_REQUEST array which contains the product_id of the current product.