Planet Money (an NPR show) created a t-shirt for fans and broke down the cost of each shirt. The biggest line item was shipping the finished shirts to customers.
http://www.npr.org/blogs/money/2013/12/13/250747279/episode-...
Out of the total cost of $12.42, it cost $2.26 to ship. It cost $1.04 to accept payments. Compare these costs with the actual cost of the product: $0.60 for the cotton, $0.40 to spin the cotton into yarn, $1.00 for sewing.
It seems unlikely that Amazon as it exists today will ever rival Walmart in volume and price since they avoid these last-mile delivery costs.
Amazon should take on FedEx and UPS and try to disrupt the industry one key city at a time. They've already started delivering groceries in Seattle, so perhaps that's the first step to Amazon Delivery Service.