The Internal Revenue Service estimates 39 percent of all individual returns filed in Missouri will be mailed in the old-fashioned way.

Though more returns are filed electronically than in hard copy, mailed returns are still expected to increase over last year, the Postal Service reports, because more Americans are expected to file tax returns this year.

"The most common mistakes Americans make in mailing their forms are not including enough postage and not providing the correct IRS mailing address," the Postal Service says. "After the first ounce – which usually covers about four, letter-sized sheets – mailers need to add 17 cents for each additional ounce."

"Post offices are not extending retail hours on tax day so customers are encouraged to mail early in the day."

Reportedly, 20 percent of all Americans wait until the last week to file their returns.

This is the third year Post Offices are not extending hours, but want to remind everyone to get forms mailed early in the day, says Valerie Hughes, spokeswoman for the Gateway District in St. Louis.

"In today’s world, our customers have many alternatives to mail things," Hughes told the Missourinet. "We have contract postal units, we have the automated postal centers, and of course, tax day comes the same time every year, so Americans can plan for it." 

To find the correct Internal Revenue Service address for your tax forms, visit the Missouri page .

Jessica Machetta interviews Valerie Hughes [Download/listen MP3]