Creating a shopping cart in Freeway couldn't be easier and can be absolutely free too. The Freeway Shop Action suite (which ships and installs with Freeway's core actions) ties in with the shopping cart service from Mal's e-Commerce. You can add links to review the cart, links to add products to the cart, and add quantity and product variations to the items you're selling.
There is a detailed tutorial on how to setup your own cart in the complete Freeway Reference Guide which you can download for free. The chapter containing the Freeway Shop Actions tutorial begins on page 403.