Almost 2 million pieces of mail were processed Monday at Chicago's main post office.
More than 600 million are being handled nationwide.
E-mail and electronic bill-paying have reduced post office business and put the service into serious financial trouble.
But, from Thanksgiving to Christmas, 16.5 billion items are likely to be mailed nationwide.
"My goal was to come a little bit early, because I figured there'd be a long line. But I didn't expect it to be this long," said Post Office customer Emily Radecki.
"This is when our customers are counting on us, to make sure that their cards and packages get delivered on time for the great day. And our employees--as they are every year--are certainly up to the task," said Mark Reynolds, US Postal Service in Chicago.
If you send out first class letters by Tuesday, Priority Mail by Wednesday, and Express Mail By Thursday, it should all arrive by Christmas day on Sunday.