If you have already received a tracking number, your order has been shipped and received by the specified carrier. UPS has a very reliable tracking system however; USPS (US Postal Service) can take some time to update.
If the package was just shipped, you may see this notice on the USPS website:
“Tracking information is not available at this time. Please check back later or double-check the tracking number entered and try again.” OR
“The U.S. Postal Service was electronically notified by the shipper on [date] to expect your package for mailing. This does not indicate receipt by the USPS or the actual mailing date. Delivery status information will be provided if / when available. Information, if available, is updated every evening. Please check again later.”
Note: USPS sometimes fails to scan packages so tracking will not activate, even after the package has arrived.