Today WebAssist announced the availability of their PayPay eCommerce Toolkit for Expression Web. After installing this Toolkit, you can easily add features such as "Pay Now" and "Add to Cart" by using the Insert/PayPal menu item that gets integrated into Expression Web's menu. After dropping the PayPal button on your web page, a wizard walks you through the steps to configure your payment (PayPal email address, shipping cost, etc). It appears to be a very useful and well written add-in for Expression Web.
It's hard to tell whether or not it's just a current promotion, but for now the PayPal eCommerce Toolkit is being offered for free. The Toolkit is available here: https://www.webassist.com/professional/products/productdetails.asp?PID=118