So let's say you order a widget from Amazon, and they have it (or quickly get it) in stock at one of their fulfillment centers only 40 miles away. They box it up and put a USPS prepaid label on it and drop it at the nearest postal facility for local delivery.
BUT… what if that label says it originated at a fulfillment center 1500 miles away? The USPS gets to charge for handling the package the entire distance. Even though they give Amazon a discount rate, they still come out ahead as they only have to do local delivery.
Benefit for Amazon? Cover for their illegal long haul transport operation.