Linda has been involved in this program for years. Her husband, Cec, worked for Canada Post.
https://lindaandcec.blogspot.com/2020/11/letters-to-santa.html
I was curious what other countries did. It looks like the US requires a stamp on the letter and also a stamped return address envelope. This year they seem to have an online system too. And some kids actually get gifts they ask for.
https://www.washingtonpost.com/business/2020/11/25/santa-letters-usps-coronavirus/
And this is what they do in the UK. They also require a stamp.
https://www.royalmail.com/christmas/letters-to-santa
I brought this up when I googled "letters to Santa around the world". Some of the links didn't work but New Zealand and Ireland did. Ireland is also kind of cool as they provide colourful templates to download. I didn't try all of them. You can email Santa in New Zealand and get a postcard from Santa from the North Pole.
https://www.postcrossing.com/blog/2015/11/25/santa-s-address-around-the-world
