Creating an 'express checkout' in Drupal Commerce
When you have a site which sells a single product the whole cart process seems all a bit too much especially as the cart adds nothing to the experience other than an additional screen and click which if anything will frustrate users.
Aten Design Group have a great little guide on the required steps to do various tasks. Although the post was written in 2011 it still applies to the latest version of Drupal Commerce.