shipping is being paid by the people who bought the cards iirc, i dont think its free/provided by the creators
Shipping the product to the customer would have the costs covered by the customer themselves.
But there is still a shipping cost for moving the product from the factory where it is produced, to the office/house/warehouse that Exploding Kittens will sell from. They have to cover that cost of shipping.
Then they sell the product to customers, and ship it off to them, with the customers paying for that.
There likely isn't any international shipping cost for the company, but there is for just having the product created and delivered to them.