Orland Park Police said the 2008 Lexus SUV drove over a parking block and into the front windows of the post office in the 9500-block of 144th Place around 1 p.m.
The vehicle struck an 81-year-old woman who was waiting in line, who was knocked to the ground. She received minor injuries and was transported to Palos Community Hospital.
The driver was not injured. Police said the driver indicated her foot slipped off the brake as she was pulling into the parking spot.
The post office is closed until further notice.