(a) Canada or (b) USA?
Assuming that Santa Claus lives at the North Magnetic Pole. The North Magnetic Pole (according to Wikipedia) was in Canada until 2017 and then moved towards Russia, which would have it moving “over” Alaska. It would take an expedition to the Magnetic North Pole to determine its exact location. Scientists are only estimating its location with math.
You can still write a letter to Santa Claus in Canada.
Address it to Santa Claus, North Pole, Canada.
Postal code is HOHOHO.