It happened around 4:23 p.m. as the 38-year-old female employee was driving eastbound on the 3900 block of W. Harrison Street.
According to Chicago police the victim suffered a gunshot wound to the back by an unknown offender.
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The woman was transported to Mount Sinai Hospital in good condition.
A USPS spokesperson said postal inspectors are working alongside Chicago police to investigate but could not provide more details.
No one is in custody.