Amazon's flying delivery drone stunt may have scored the company a publicity coup on Cyber Monday Eve. But for a truly radical vision of the future, imagine streets crawling with another kind of Amazon machine: grocery trucks.

According to various reports, Amazon will soon announce that San Francisco is the latest city to offer Amazon Fresh, the grocery delivery service the company is slowly rolling out across the country. In fact, we spotted a truck yesterday. (Update: Amazon Fresh officially launched in San Francisco today.)

Amazon spent years experimenting with same-day grocery delivery in its home city of Seattle before rolling out the service in Los Angeles late last spring. For a fee of $299 per year, Amazon Fresh promises unlimited same-day or next-day early morning delivery of more than 500,000 items, including groceries.

>'They take packages, and they take packages back. They control the entire infrastructure' Ajay Agarwal

But the most important thing isn't what Amazon is delivering. It's how Amazon is delivering – using trucks that belong to CEO Jeff Bezos and his company.

Ajay Agarwal knows Amazon. As a managing director with Bain Capital Ventures, he led a big investment in Kiva Systems, the warehouse robot company that Amazon paid $775 million for last year. Agarwal says that Amazon may be taking an ever-greater chunk out of the world's brick-and-mortar retail sales, but physical stores still have Amazon beat in one key area. "What's the biggest negative of Amazon? Returns," he says. "It's a royal pain...I feel like a daily, weekly exercise for me is breaking down boxes, doing returns, printing out return labels, etcetera, etcetera."

But a dense network of Amazon delivery trucks could make returning unwanted items as easy as taking out the garbage. Unlike electronics or books, which most people shop for sporadically, grocery shopping takes place regularly and often. If Amazon Fresh takes off, that will mean frequent, predictable trips by Amazon trucks down residential streets. For every grocery order delivered, those trucks will have room for another return. "They take packages, and they take packages back," Agarwal says, much like the milkman who in distant days not only delivered your milk but also picked up the empty bottles. "They control the entire infrastructure."

That deep control has been a signature element of Amazon's operations, from the first website visit to the moment an order leaves a warehouse. But that's when Amazon hands off that order to a third-party carrier, typically UPS or FedEx. Such a concession must drive a control freak like Amazon CEO Jeff Bezos nuts – even if those companies have helped cement Amazon's reputation for reliability by delivering on Amazon Prime's promise of two-day shipping. With an army of its own trucks on city streets, Amazon cuts out the middle man. Meanwhile, returns could become as easy as handing a box back to the same Amazon driver who brought it in the first place.

"In the old days, it used to be your milkman coming to your house every week," Agarwal says. "I think in five years I could imagine...some significant fraction of the population having an Amazon truck coming to their house every week."

This reality, he believes, is "way closer than drones."

But, he adds, Amazon must tread lightly. For now, the company depends intensely on UPS and FedEx to make its business work. At the moment, Amazon can't make them angry. It's telling that a day after Bezos revealed Amazon's flying drone ambitions on 60 Minutes, which would also be a form of direct shipping, news leaked of UPS' own drone plans, as if the delivery company was saying: "Don't test us."

Though emphasizing he doesn't have any inside knowledge, Agarwal says the time could come when Amazon and its key shipping partners become "mortal enemies." Amazon, he says, may already be hedging against such a possibility.

Witness Amazon's deal with the U.S. Postal Service to have Prime packages delivered on Sundays. As long as the Post Office stays in business, Amazon has a shipper that's legally obligated to deliver its packages, even if its deals with UPS and FedEx fall through. For anywhere its own trucks don't go, Amazon will still have the mailman.

"I think what's going to end up happening is Amazon is going to cherry pick the best routes, the most dense communities with their trucks and Amazon Fresh," Agarwal says. "That random person in the remote area? U.S. Post Office."

In theory, a more powerful drone – with a much greater range – could also handle deliveries in the hinterlands. But the real issue isn't really between air or ground. The issue is whether the trucks will have drivers.

"To me, would I bet on self-driving cars delivering packages or drones?" Agarwal asks. "I'd bet on the cars first."