The crash happened around 9:30 a.m. Tuesday.
In the video, a blue car is seen clipping the back of the mail truck, causing it to flip.
Neighbor Doug Neumann said he was startled when he saw the scene. His knee-jerk reaction was to jump in and help the mail carrier.
"I ran over there and jumped inside, and - or jumped on top of it and helped the lady out," said Neumann.
Neumann said he did it all with a recently operated on foot and in flip flops. The former Waukesha County firefighter said his instincts took over.
The driver of the mail truck was injured but expected to be OK.
RELATED: Arson investigation: Contractors rescue child, woman and dog from Michigan City, Indiana house fire
Police said the driver of the blue car meant to hit the brake but hit the gas pedal instead.
USPS sent CBS 58 this statement:
"The accident is being investigated by local authorities. Privacy regulations prevent the release of employee information."
Waukesha police said the elderly driver will most likely need to take a driver's retest.
The-CNN-Wire & 2023 Cable News Network, Inc., a Time Warner Company. All rights reserved.