They actually tell you to mail it by UPS only. I mailed it by USPS first and they lost it en route. I wrote my congressman and practically begged him to help. He contacted the postmaster general's office who told him that it was essentially lost for good even though the USPS was "searching for it" and that there was a 0.01% chance I'd ever see it again. My rep then contacted the state department who printed me a new passport in 48 hours, which is basically unheard of. Mailed it again by UPS and got it back just in time to leave for the UK. God what a nightmare, I am lucky to have had a great representative at that time. He really went to bat for me.
My other passport is still lost, lord knows where.