That is so dumb! As I said in my response to Lynds, nowadays cards have all sorts of stuff attached to them - including hard plastic. It's so arbitrary.
I even was thinking about those cards with seeds in them - if I bought a seeded wildflower card here in Colorado and sent it to Canada, some of those seeds might be invasive to Canada, but they're not going to stop the card, are they? No, they're not. Because it's paper, as far as they care.
I've even put cat toys inside cards before - little, hard plastic cat toys - and sent it through the regular post. Nobody stopped it.