Is sending a homemade postcard illegal?

I'm in Canada. I don't see why it would be illegal, I'd just make sure to make it the same size as regular postcards or a bit larger, they do have oversize postcards, and from heavy enough card that it won't crumple in processing. My vet sends reminder cards that are really just heavy paper, not even cardstock. Years ago I was a letter carrier, and I delivered a series of cards to one house from some of their friends who were travelling. They were polaroid photos with stamps on.