"Operation Santa" has been going on since 1912, giving gifts to needy families from anonymous people. But with the downturn in the economy, less people can help, leaving thousands of families with nothing to open on Christmas.
The letters are written by families struggling this holiday season.
"Letters that come from kids and letters that come from parents that are experiencing hardship," said USPS communications specialist Robin Anderson.
There are 12,000 letters right now, and more coming.
Among the letters asking for fancy electronic items are letters asking for things families should not be without. That's why Rosemarie Ansell has been responding with gifts and even food for almost 15 years.
"I feel, if I put a little money aside, I can make a kid's dream come true that I'll receive back. I believe what goes around comes around," Ansell said. "They ask for simple things, like food, socks or shoes, that we take for granted."
But, with so many people struggling, not everyone can give this year.
Eight-hundred people have come to answer letters, down 400 from this time last year. The Post Office is pleading for help.
Veronica Payne has been on both sides. After helping two years ago, she struggled financially last year and couldn't answer a letter. But she is back again this year, ready to help three children with their lists.
"I firmly believe if you're in strain, or if you don't have the best way for you to be blessed or be fortunate, is to give to someone else," said Payne.
To protect the privacy of families, you can drop off the gifts at the Post Office and they will send the packages out in time for Christmas.
If you're interested in picking up a Santa letter, log on to the USPS website: http://about.usps.com/news/electronic-press-kits/holidaynews/2011/santa.htm.