Hi,
Adding a button in the toolbar can be done by creating a small plugin of the group HikaShop implementing the onHikashopBeforeDisplayView function trigger, with something like that:
function onHikashopBeforeDisplayView(&$view){
if($view->ctrl != 'product' || $view->task != 'show') return;
$this->toolbar[] = array('name' => 'Popup', 'icon' => 'send', 'id' => 'send', 'alt' => JText::_('HIKA_EMAIL'), 'url' => 'index.php?option=com_hikashop&ctrl=order&task=invoice&tmpl=component&type=full&order_id='.$order_id.'&custom=1', 'width' => 720);
}
This will also open a popup with the same content as the one when you click on "invoice" but with an extra parameter "custom" set to 1.
So then, you can edit the file "invoice" of the view "order" via the menu Display>Views and use such check in there to have your custom text for that new button:
if(@$_GET['custom']){
// echo my popup content
}else{
// echo when it's an invoice
}