This is what I speculated above. Basically AWS for logistics. It can't be far off in many countries such as USA/UK
You can do FBA and ship orders that were not purchased on Amazon.com. I have no idea if it's pricing is competitive with other ways of doing this. A big selling point in using FBA in the first place is that's what Amazon.com customers want. Without that big piece in the equation it might not be worth it to use Amazon's fulfillment centers, at least not at whatever price they're charging.
They should do a human-sized version of Amazon Snowball.