When USPS tracking says Pre Shipment it usually means that the person shipping the package has created a shipping label and has paid for postage but the person still has the package in their possession and hasn't handed the package over to the USPS yet.
When the package gets to USPS and is in USPS possession then the tracking should update to accepted at "facility" or something similar since sometimes USPS uses different terms on their tracking information.
The Pre Shipment in simple terms means that the package is getting ready to be shipped and handed over to USPS.