I've had packages through USPS stay stuck on Pre Shipment until the package was actually delivered to my address.
Sometimes USPS doesn't scan all the packages like they should be doing when the packages pass through the sorting facilities so it's possible that your package was just not scanned after the package was sent to the USPS or picked up by the USPS.
I wouldn't worry until a week after and if it hasn't updated from Pre Shipment after a week and you still haven't gotten the package then contact the seller who shipped you the package and also contact the USPS so they can track your package through GPS.