I developed something similar. I created a module activated by cron (once a day) that checks the last orders that have 'delivered' (complete status).
To run a function in your cron magento module, add to your config.xml
<crontab>
<jobs>
<myModule_myFunction>
<schedule><cron_expr>*/60 * * * *</cron_expr></schedule>
<run><model>emailtracker/cron::myFunction</model></run>
</myModule_myFunction>
</jobs>
</crontab>
For the order to have supplies you can use the following code snippet
$orders = Mage::getModel('sales/order')->getCollection()
->addFieldToFilter('status', 'complete')
->addAttributeToFilter('created_at', array('gteq' => $dataForFilter));
Note that the collection of orders that I created, I make filter by date to avoid retrieving all orders already made.
With my collection of recent and complete orders that are possible candidates to receive a tracking email delivery, I can actually work with the orders that are important.
I created a foreach to run in my collection and within that loop, retrieve the tracking number.
foreach($orders as $order) {
$trackings = Mage::getResourceModel('sales/order_shipment_track_collection')
->setOrderFilter($order)
->getData();
for($i=0;$i<count($trackings);$i++) {
$trackingNumber = $trackings[$i]['track_number'];
//Make your php magic here
}
}
From this point, you can work this information the way you want. You can create a function that checks the status of tracking number via API and its delivery service sends an email, or you can imagine.
Important: Remember to check if your magento cron is active.
Note: I did it in Magento 1.6.1.0