We have found that when we're selling a single product, a landing page that includes the registration / payment form is the most effective. Here's an example.
https://www.kiplinger.com/orders/krpg/index.html?source=shopsite
I have been told that it's impossible to make something like this work in Shopsite because there isn't any way to bypass the checkout process. I think that would be a good enhancement.
The Shopsite cart is very nice, but there are times when you just want a one and done page.
Thanks,
Greg