Zadrs, you'll just want to modify admin/invoice.php to show the product id. For example, this code puts it right after the model number.

Change admin/invoice.php line 179 from

Code:
           '        <td class="dataTableContent" valign="top">' . $order->products[$i]['model'] .  '</td>' . "\n";
to

Code:
           '        <td class="dataTableContent" valign="top">' . $order->products[$i]['model'] . " (" . $order->products[$i]['id']. ") ".  '</td>' . "\n";