You can find the tracking number and status of a package on the Shipments page.
The number and a short description of the status (for example, “In Transit” or “Delivered”) will be listed under the “Tracking Status” column. If you click on the tracking number, you will be directed to the carrier’s tracking page for that shipment, where you can get more information about its status.
Tracking is available in Shippo with all carriers for all service levels where it is offered by the carrier. Tracking is available for around 98% of service levels. There are no additional fees for tracking.
USPS First Class International shipments do not include reliable tracking. Depending on the country, you may get some (very spotty) tracking information, at least until the package leaves the US. But in many cases, tracking is not available for these shipments. USPS Priority Mail International Small Flat Rate Boxes and Flat Rate Envelopes also do not include tracking (see: https://www.usps.com/international/priority-mail-international.htm).
Looking for information on the Shippo tracking API? Tracking API documentation.
If your tracking is showing as "Invalid" or "Unavailable" when you check it on the carrier's website, it's likely because you've only just created the label. It can take 24 hours after label creation before tracking information is processed by the carrier's system.
USPS also reuses tracking numbers and so it can take 24 hours for their system to update with the correct shipment information (address, status, etc.).