Google now supports embedding a product checkout link on their listings: https://support.google.com/merchants/answer/13580732?hl=en

Using the checkout feature, you can make the shopping experience quick and easy for customers by giving them the option to go directly to your checkout landing page or cart from your free listings. One way you can do this is to provide a URL template of your checkout or cart page at the account level. Another way is to enable checkout at the individual product level by providing the checkout link template [checkout_link_template] attribute in your product feed.

The cart software needs updating to support the product id in the link. i.e. cart.php?item_id={pID}

John