Here’s a nice one under the FAQ section of the USPS website: Writing Letter to Santa Claus or God

From the FAQ:

Santa enjoys candy canes and cookies, and his reindeer enjoy fresh hay. But because these treats can be damaged or crushed when sent in a regular envelope, Santa says it’s better to leave them out on Christmas Eve than to mail them.

Cause most kids go out an buy hay for the reindeer.

Like all mail, it’s important that letters to Santa include a complete return address with ZIP Code.

Train em while they’re young!

Letters to God can be addressed in the same way replacing “Santa Claus” with “God”.

Good to know that Santa Claus and God are interchangeable. That should clear up a few theological debates.

Of course, there is a serious side to this.

While Santa receives most of these letters addressed to him at the North Pole, AK, some are sent to certified organizations and individuals who help those in need. Many Santa letters will be answered by a variety of Santa’s helpers – charitable organizations, employees of local Post Office™ locations and volunteers who want to help make children’s holiday wishes come true.

So yeah, there’s some people out there who are jolly and good-willed and all that, and they write back as Santa Claus.

No word on who returns mail sent to God.