If you want to express mail a letter to a post office box, your options are limited. The United States Postal Service is the only overnight mail service allowed to deliver documents or packages to a post office box. You can use the Priority Mail Express service to get your letter into the recipient’s box overnight 365 days a year, though delivery on Sundays and holidays costs extra.
The USPS prices its Priority Express Mail service based on how far your letter is going. The USPS divides the country into nine zones, and the movement between zones factors into the pricing. So does the size and shape of the parcel. For a letter, a flat rate envelope or legal flat rate envelope will be sent at the same price regardless of how many papers are stuffed in there. The postage price calculator on the USPS website (http://postcalc.usps.com/) lets you know how much the delivery will cost. Priority Express Mail comes with delivery tracking so you can confirm it arrived as scheduled. If it hasn’t arrived by the deadline promised, USPS will refund the shipping costs. "Delivery" means it was placed in the desired post office box – not that it was actually picked up.