This year marks the 101st anniversary of the Letters to Santa program run by the USPS. The program relies on a network of post offices and public participation to help Santa answer the letters that thousands of children mail to him every year. Here's how it works (from the USPS site):
Individuals who would like to adopt letters are asked to do so in person, present valid photo identification and fill out a form that includes the list of letters they are adopting.
Here’s how it works. A person wishing to adopt a letter can go to a Post Office, select one or more letters to take with them and sign the form. The child’s address on the envelope will have been blocked out and the letter assigned a number.
After the individual fulfills the child’s wishes with a gift, he or she returns with the letter and gift to the same Post Office and pays the postage for the package. A postal employee will match the number on the letter with the child’s address, apply a label to it and put the package in the mail stream.
Here is a handy link to the list of participating Post Offices. If you're still confused or need a little more inspiration, take a look at how Jennie of Red Letter Day gets her Santa's helper hat on. She's an expert elf for sure!