That's why market estimates for online retailing are completely wrong and always have been. Although the boundary isn't fixed, it isn't all that elastic either. An e-tailer's business processes are those of a cataloger. It's a very different discipline. Having a warehouse doesn't make you ready to offer customers online shopping carts.
Returns are just one reason, but an important one.
A good place to start is Paco Underhill's book, Why We Buy. As you read it, I expect you'll find lots of notions you can use in your own business. Among those notions will be many examples of how limited a role IT has to play when a business meets its customers. That's a subject we'll explore next week. Until then, go shopping.