It happened at approximately 2:15 a.m. along Interstate 495, north of Edgemoor Road.
Investigators say a silver 2009 Nissan Altima ran into the rear tires of a USPS truck. The truck then reportedly hit a guard rail and overturned down an embankment.
Authorities say the driver of the USPS truck -- a 59-year-old man from Yeadon, Pennsylvania -- died at the scene.
His identity has not yet been released.
The 31-year-old man driving the Nissan was taken to a local hospital and is expected to be OK.
The road was closed for nearly seven hours while police worked to clear the scene.
Authorities are still investigating what caused the deadly collision. Anyone with information is asked to contact the police at 302-365-8486.