The fire broke out around 1:30 a.m. in an electrical supply store located above the post office on the second floor of the building on First Street.
More than 10 fire departments responded to the scene of the fire, which was brought under control around 4 a.m.
No one was inside at the time.
The fire was mostly contained to that top floor, which suffered heavy damage. Most of the damage at the post office was from all the water dumped on the fire.
One firefighter was taken to the hospital after he fell from the roof, prompting a switch to an external attack. A second was treated at the scene for minor injuries.
"Upon arrival, we had heavy fire on the second floor, a partial collapse from the roof down into the second floor," said Dennis Monks, with the Carlstadt Fire Department. "We started an interior attack. Due to the safety of the firefighters, we had pulled them out. We went to an exterior attack."
Firefighters were still checking the building for hot spots.
Mail was being removed from the building and put on trucks, but it wasn't clear if any mail or packages were compromised.
Inspectors were on the scene to determine the structural integrity of the building, which was believed to have been compromised.