In Google Search settings -> Google Now, I have "Show cards based on Gmail" checked. "Packages" is on, and the description says "when an order confirmation email is delivered for an online purchase"
Since turning that on, I've used mutt's bounce feature to deliver two Amazon shipping notices to my gmail account. The email notifications showed up on my phone, but (even a day later) I still haven't been able to get the Google Now package tracking card to show.
How do I get it to work?
Here's the message in gmail:
I'm now on Android 4.2, and (after the 4.2 upgrade) received an email from the Google Play store with a tracking number this morning. Still no delivery status card from Google Now. Will check the Amazon soon again, too (once they ship me something).
I've got a Nexus 10 as well now (also running Android 4.2, stock, not even rooted), and package tracking also doesn't seem to work there, either.