Direct checkout button not working for vitiable products

Hey, thanks for this really simple and nice plugin!
It works as expected for simple products, but on pages with variable products, the button doesn’t add the product to the cart and takes the customer to an empty cart page.

Is this expected behavior? Or is it some bug?

Awaiting your response,
Achyuth.

The page I need help with: [log in to see the link]