Hi,
We didn't do a plugin for that as we didn't think that it was really useful. When you edit a product in hikashop you have the button "add to cart HTML link". Upon clicking on it, a popup will open displaying you the HTML you can use to display a link which will add the product to the cart and direct to the checkout automatically. Basically, it's just the URL http://your_website.com/index.php?option=com_hikashop&ctrl=product&task=updatecart&quantity=YY&checkout=1&product_id=XX where XX is the ID of the product and YY the quantity to add.