While it can be alarming to see a premature Delivered status on a package you have just printed a label for, it usually isn't cause for concern. This is common with USPS tracking numbers.
There are two common reasons that a USPS tracking number might reflect a Delivered status, even though you just generated a label or shipped the package. The USPS may have assigned a recycled tracking number to your shipment or the USPS scanned the shipment incorrectly.
To check if this is the case, look up the tracking number details. 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.
This situation may happen because of the high volume of shipments sent through USPS. Tracking numbers get recycled on occasion. The solution to this issue is to wait until the USPS 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 USPS. 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. A USPS customer service representative should be able to provide you with updates via their internal tracking database.