It depends on the branch. The central post office in Chicago, at Polk and Canal, is now the only post office in the U.S. with 24-hour service, though, so you can always mail something there anytime.
Old Chicago Main Post Office was created in 1921.
Chicago, Illinois.
Contact them and ask.
The phone number for the downtown Wheaton Post Office is 800 275-8777.
Clause 7
Chicago is the location of only post office in the world through which you can drive your car.
MEOW
Ask at a post office in Chicago.
Just one stamp 42 cents for America. You can always call your post office in Canada there. they will know, just to make sure.
Chicago's O'Hare International Airport is located on Bessie Coleman Drive in Chicago. Their mailing address is directed to a post office box because of the volume it received every day. They can be contacted via mail at O'Hare Airport, P.O. Box 66142, Chicago, Illinois, 60666
There is no US Post Office with that name and so no zip code.
'Sources and related links' has the current rate (which was 9.5% at the time of this post)