Hi Diana,
My fellow Disney Parks Moms Panelist,
Amanda. F, provided a great answer to this very question from a Guest just last November - you can read her whole
answer on the Disney Parks Moms Panel website.
Coconut postcards are indeed available to purchase from the Calypso Trading Post at
Disney's Caribbean Beach Resort.
The cost of the Coconut Postcard is $19.99 (which is of course always subject to change) and this does include Priority Mail shipping through the US Postal Service.
With regards to shipping a coconut postcard to Canada, Amanda F was told by a Cast Member at Disney's Polynesian Village Resort that this was possible although additional shipping charges would apply.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma