I believe you are right, I believe that is part of the Amazon/DHL joint adventure. DHL is international only, they are not licensed for domestic Air-cargo. Amazon now has there own Air-cargo hub for domestic services.
An Amazon fulfillment center is coming to an area near you...
http://aircargoworld.com/amazon-makes-major-bid-to-boost-european-warehouse-market-share/
BTW, the owner of Amazon started off by selling books out of his garage boxing and shipping them himself...