Hey there Aaron!
Cynthia here, a Guru with Shopify :).
That is something that you can change/add while you are adding tracking to an unfulfilled order, and you can also modify the tracking on existing orders.
For example, on one of your most recent orders, if I click More->Edit Tracking (inside the Fulfillments box on the order page), a dialog comes up showing the China Post tracking information:
You can switch that over to USPS via the tracking URL box, or the Carrier box, as USPS is listed there.
The same tracking number/carrier fields show up while fulfilling an order as well, so there's a place to change that before and after the fact :).
I hope this was helpful - if you need anything else, let me know!
Cynn | Guru @ Shopify