Some people do not know until they are about to travel that their passport is expired or about to expire. So this puts them into the category of "urgent need of a passport". To prove this, you have to submit a confirmed itinerary along with the rest of your documents. In the case of expedite processing, USPS is not always reliable.
The following link goes to the US Department of State. http://travel.state.gov/passport/get/fees/fees_3259.html They are issuing apologies and refund offers to people who used the USPS for expedite processing but ended up missing their trip due to mail delay. To work around this, you can hire a passport courier service that is registered with the US Department of State Passport Agency.
Passport courier services deliver thier clients' applications in person to the Passport Agency and are given pick-up receipts for a specific date. When that date arrives the passport is released to the passport courier service. The service you hired immediately uses FedEx Overnight to ship your passport to you so you do not end up missing your trip. Processing times vary by price. They can be as fast as 1 business day or as long as 12-15 business days. www.Direct-PassportVisas.com.