What USPS service levels can I get tracking information on?
You can get tracking information on all USPS service levels EXCEPT USPS Flats and Letters. USPS doesn’t support tracking for Flats and Letters.
If you absolutely need tracking, you’ll need to use Parcel as the
predefined_package for your USPS Flats and Letters shipment. This will obviously cost more, but you will get the full tracking of the shipment in transit.