Priority mail only gets useful the further you get away from the place it's being sent from. Sending something priority from NYC to Philly won't save any time compared to sending it first class, both will usually arrive two days after it was sent. However, sending a parcel from NYC to LA by priority is where it has it's usefulness, in that it is garunteed to reach there within 3 days, where first class can take 4-5 days. The other thing with priority is that you can track them with delivery confirmation, something that's not available if you use just first class. I don't believe you have to sign for priority mail, just express mail which is garunteed overnight. As for anything reaching anyplace within the same day, that's not going to happen. The best one could hope for is next day. For same day delivery, you'd have to hire an courier service. There are a number of indipendent companies who do this, and Fed Ex and UPS all have a few services like that, but it's all very expensive.