CHICAGO (CBS) — The Christmas rush has hit the post office.

The next couple days marks the busiest days for those looking to mail packages and greeting cards. WBBM’s Michele Fiore reports.

More than 612-million pieces of mail are expected to be processed nationwide on Monday, according to the U.S. Postal Service.

By far, the most popular question at the Aon Center’s post office on the Monday before Christmas is, when will it arrive?

“Nobody wants to wake up Christmas morning and what they thought was going to be there is not.”

Tanya Watts, was happy to learn first class mail still has an expected arrival ahead of Christmas, but Janelle Barnett was thinking of spending the extra money to be extra certain.

Tuesday officially marks the Christmas deadline for those shipping domestically using First-Class Mail and internationally using Global Express Guaranteed.

For the procrastinators, the Christmas deadline for shipping Priority Mail is Dec. 21st and shipping Priority Mail Express the deadline is Dec. 23.

Those using the Aon Center’s post office around lunchtime Monday got a lesson in patience.

“I figured this is how my lunch would go, so it’s okay,” said Caroline Trudeau. “I anticipated it.”

Trudeau’s letters are among 612-million items the U.S. Postal Service expects to process on this, the busiest mailing day of the year.

Jenna Smith passed time in-line, playing games on her phone.

“It took me maybe 20-25 minutes,” Smith said. “I think it’s pretty good.”

To avoid any shipping issues, the U.S. Postal Service has provided a list of tips to make the holidays go smoother. Here are some of those tips:

– Print mailing and return addresses clearly
– Pack gifts and boxes properly
– Remove batteries from toys
– Never guess a zip code

USPS also warned of not sending cash in the mail, as checks or money orders are safer options.