YES Enclose That Checkout
As a Director of Ecommerce I came to think of checkout as similar to a plane landing. When a plane is landing its wheels are down, flaps set and they are coming in UNLESS there is a distraction or a danger. 

Checking out should be like landing a plane - clear, well mapped and without distractions or dangers. Enclosing the checkout reduces distractions. 

You worked so hard to get your customers into a checkout process you should ENCLOSE the process by reducing or eliminating upsale, navigation and other distractions and this post shows you how.  

Work Backwards
Most website designers work IN to the checkout. This means they are spending most of their time on pages that are important but less important than the checkout. 

Check is where ecommerce rubber meets road. If you create sense of security and ease people remember and return. If your cart is complicated, clicky-y and a pain they don't. I typically pick these "how to" posts apart because they over simplify and so kill the real issue of commerce online. 

This post is solid, shares great tips and knows what it is talking about (no nits from me, stop the presses LOL). If you are creating an ecommerce store, and everyone should have one, follow these tips to create a solid checkout experience. 

Not a bad idea to work backwards. Create the cart first and let that design set the clear and simple tone for the rest of the site. 

Read my 5 New Ecommerce Lessons for why everyone should have an ecommerce store:

http://scenttrail.blogspot.com/2013/06/5-new-ecommerce-lessons.html