When the Status of USPS tracking states that Origin Post is preparing shipment it means that the origin post is preparing to dispatch the shipment to the USA if your package is coming from another country other than the USA.
If your package is coming from the USA and is going to another state in the USA then it could mean that a shipping label was created but the USPS hasn't gotten the package yet or they're preparing the shipment to be put on the truck to send it on it's way to you.