USPS Tracking service provides end-to-end product tracking and is easily available for most domestic email products which are large enough and which are addressed to national places, for example to restricted Army Post Offices (APOs), Fleet Post Offices (FPOs), Diplomatic Post Providers (DPO) destinations and also to ZIP Codes in U.S. lands and possessions or Freely Associated States.
Going into the tracking number related to a mailpiece into the USPS Tracking page gives you the current status of a product along with other tracking information. That info includes, if available, shipping and/or attempted delivery information, such as date and time of delivery as well as the shipping location of this thing (mailbox, reception desk, neighbor, etc.).
USPS Tracking doesn't guarantee or expedite delivery. There's no excess cost for the tracking mechanism on products that automatically include USPS Tracking. Basically, you cover for that product's postage, and monitoring is included, unless you are sending USPS advertising Mail® parcels.