When tracking info for USPS says Pre Shipment it means that a shipping label has been created and postage has been paid.
The USPS may or may not have the package in it's possession yet but usually when USPS gets the package or picks the package up the tracking information should say package acceptance pending or that they have the package and it's on it's way to a USPS facility.
Sometimes USPS tracking does not update like it should if the package does not get scanned by a postal employee.
I've had packages that are shipped through USPS stay stuck on USPS pre shipment for 4 days until it reached my post office and then it updated to being delivered.
So I would not worry about the package until 4 to 5 days after it has been shipped.
If you have not gotten the package within 7 days then contact the shipper who shipped the package to you.
Some shippers such as sellers on ebay may take a few days to ship out a package but they may have already created a label.
Just remember though that USPS tracking information is not really that reliable.