One of the first things we do when we start working with a new ecommerce clientis check out the shopping cart. In general, they are much better today than they were a few years ago. However there is usually lots of room for improvement.
One of the best ways to demonstrated the problems is to do ausability test and show the video to the shopping cart owner and development team. That is usually enough to convince everyone there is a problem.
However now there is a shorter way. If you need to convince people to improve their shopping cart, try showing them this parody of online shopping as if it was done in real life.