One of the first things we do when we start working with a new ecommerce client is 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 a usability 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.