These are the mail carrier API's we support:
UPS (Surepost, UPSMI)
USPS (Blue Pkg, Smart Mail, Parcel Pool)
Fedex (Fedex Smartpost)
We have some additional logic; if a tracking number from an unknown carrier "looks" like certain ones, we will try the known carrier as well. For example, if a tracking number starts with 1Z, we will check UPS.
Also, for carriers that we do not yet directly support, our system will essentially just "trust" that the Estimated Arrival Date that Amazon provides for FBA orders, and then assume that the package is delivered that day. Estimated Arrival is only supported for FBA Orders. If the mail carrier you use is not supported, we recommend setting up your messages to send on the Order Events of "Shipped."
We are continuing to integrate with additional carrier APIs as needed. If you have any questions or concerns about mail carriers that we support, please don't hesitate to reach out to our Customer Success Team for more information.