I know this is an old thread, but I did come up with a workaround for this. I don’t know of a way to add it to the default woocommerce order complete email, but you can easily add it as an order note that the customer can see from the my account -> view order page, and they will also get an email.
So what ends up happening, is a customer places an order, easypost updates that order with an order note, and the customer gets two emails: one confirming their order, and one notifying them that a note has been added to their order, which will show their tracking number.
Add this code to easypost_shipping.php after shipment->buy
$order->add_order_note(sprintf("Track Your Package: '%s'", $shipment->tracking_code), 1);