While Amazon has clearly not taken delivery of all 100k vans, they may also not want to do so at the rate Rivian will be able to deliver them soon. If Rivian scales to the point where they are delivering 10k (or 20k or 30k) vans a year, will Amazon want to deal with that? It's certainly not cheap to buy that many vehicles all at once. Plus it's Amazon's first delivery EV, so they're going to need to be installing a LOT of charging infrastructure.

Amazon also wants Rivian to be successful, as AMZN has a 17% stake in RIVN. If they can get a few deals from other major delivery chains, that would do a lot of help both their revenue (because they'd be selling for more than whatever Amazon agreed to pay) and forecast. UPS, Fedex, DHL. Not to mention hundreds of regional and local delivery companies. You could probably also find a lot of smaller groups that do their own delivery (ex Imperfect Foods) and own their own vans, that might be interested in going green ( ex https://blog.imperfectfoods.com/our-commitment-to-being-a-ne... ).

Worse if Rivian doesn't start to be a player in the EV Cargo space, the market will quickly be completely absorbed by the number of other players who are in the space and actively selling into it ( ex https://www.youtube.com/watch?v=QEKMvZNkWCQ & https://www.youtube.com/watch?v=lJRNceHBc4o )