Er, to be explicit: Express Shipping (domestic) is the USPS 1-2 day overnight option.Priority is the standard 2-3 day shipping option.Only express shipping has tracking. Priority you can pay a little extra and get a DCN, a delivery confirmation number. That is not a tracking number, it's updated unreliably and is only to be relied upon to prove delivery of the package, not route or arrival times. Sometimes it really doesn't even exist until it gets put in after delivery.Express International, is yet a third thing. Which is its own beast, and is not to be confused with USPS Express domestic.So. Priority? Relax, it's a DCN. No worries.