Mailing Help. x_x?

Okay, I probably live under a rock. Because I've never sended a package without the help of some one. Ahem.. moving on. How much is the estimated cost, of a box, that weighs about a pound, to send half-way across the US? & if it was to arrive within four days. At the post office, do they have boxes, which you can.. uh buy.. ?[8x8] If so, how much would it be? Any other information is appreciated from my clueless self. Thank you.. *hides under rock*