There are a couple of reasons that a USPS tracking number might display a Delivered status, even though you just generated a label or shipped the package. It could be either the USPS assigned a repeated tracking number to your shipment or the USPS scanned the shipment incorrectly.
To check if this is the case, look up the details of the tracking number. Usually, you will see a different destination and delivery time when compared to your label details. There may also be a note in the tracking information stating the label is "Archived."
Because of the high volume of shipments sent through USPS, tracking numbers are repeated on occasion. The solution to this issue is to wait until the Post Office scans the label, then the tracking information will update accordingly.
If the tracking information has the correct address, then it is likely a clerical error on the part of the Post Office. If their scanner was set to "Delivered" instead of "Accepted" during the scan, it will update the tracking information with the Delivered status.
Unfortunately, this does mean that the tracking number will not update in subsequent scans of the label. However, a USPS customer service representative should be able to provide you with updates via their internal tracking database.