It happened around 11 a.m. at the post office at 3906 Oakland Avenue in St. Joseph.
"If this would have been on a Monday, it could have been a lot worse," Postmaster Tara Jarrett said.
Jarrett said on Monday, people usually line that back wall waiting in line. Only seven people were inside the building when it was hit.
No one was hurt when the driver of an F-150 pickup, Ronald E. Wood, Jr., 83, backed into the north side of the building, smashing the lower half of the wall. The impact broke two windows as well.
A police officer at the scene said the elderly man told him he got confused and hit the gas pedal instead of the brakes. The officer said no tickets will be issued.
The post office remains open and fully operational.
Insurance will cover the repairs and Jarrett said the building is leased.