This seems like it is a great idea for Amazon, regardless of whether this deal works out for them. If Rivian manages to build and ship these cars on schedule at the expected cost, that would of course be a great outcome for Amazon. But it's not actually necessary for the overall strategy to work for Amazon.

Suppose that does not happen? Their goal with this is simply to shift the fleet of vans they rely on today to electric to reduce cost. Currently most of that is third party operated and diesel powered. The cost of that diesel is a big component of the delivery cost for Amazon. Besides, lots of cities relevant to Amazon are currently considering diesel bans/restrictions. So they have a solid motive for wanting to address that.

The mission of these delivery vans is very simple. They do nothing else than drive to and from Amazon's massive distribution centers to pick up loads of packages and distribute them in the area. I don't have numbers here but I imagine the vast majority of these trips are well below 150 miles.

So, the mission for an electric delivery van would be to drive these routes and charge quickly in between from a preferably cheap power source. It so happens that these distribution centers are huge and vans have to stop there to take new packages on board; which represents a great opportunity to plug in.

Huge distribution center here means lots of room for a couple of mega watts worth of solar panels on the roof to power a couple of tens of charging points. The resulting power can be delivered to super chargers at the loading docks and the vans can probably suck up enough power while they are loading to cover the next round. And of course, using solar power probably gets Amazon access to some nice subsidies and grants as well as lots of great PR about how green they are.

Why partner with Rivian for this? Well, Amazon is faced with a market where the existing manufactuers are dragging their heels a bit. There are some electrical vans on the market but not quite at the scale needed and most of the big manufacturers are not shipping them in meaningful volume yet. Nothing like a Tesla like small startup to shake that up a little. Best case Rivian actually delivers. Worst case Amazon just buys electric vans from somebody else (whomever steps up). Also, they just put all the third party delivery companies on notice that Amazon wants to go electric. They too would need to charge these vans and I bet they can do that on favorable terms at Amazon's chargers in Amazon's distribution centers. Yep, they'd be buying electricity from Amazon to deliver for Amazon. Either way, the vans become electric and Amazon has a need for charging infrastructure.

Best case of course this works out for Rivian as intended and Amazon gets a nice return on investment. From their point of view, they already have a great drive train for a ridiculously overpowered pickup truck and they are already building production capacity for that. Delivery vans don't need four wheel drive and probably also don't really need 400 miles of range. Meaning, a smaller variant of the drive train but with the fast charging capability would be perfect for one of these vans. Bonus points for making them capable of autonomous driving; which is something Rivian is investing in.

Finally, why 100K? Very simple. Amazon has hundreds of distribution centers serviced by hundreds of vans. If you want to electrify all/most of them you'd need about 100K vans. Ten years is reasonable timeline to electrify the delivery fleet one way or another.