Many major post offices act as "acceptance agents" for passport applications for the State Department. The Postal Service gets a fee of $25 per application. There's a webpage at the Postal Service http://www.usps.com/passport/welcome.htm?from=home_wouldyouliketo&page=applyforpassport that has more details.
One suggestion: don't bother calling the Postal Service's telephone help line for anything dealing with this service; my experience with trying that was highly unsatisfactory. It worked out better for me to simply visit the passport desk at a large post office.
In fact, don't bother calling the Postal Service's telephone help line for anything at all. It is beyond useless.