Amazon has been pushing many of its larger Amazon FBA merchants to use Seller Fulfilled Prime (SFP). Instead of these merchants shipping from their own warehouses to an Amazon fulfillment center (FC), merchants ship directly from their own warehouse to customers. Amazon is dangling huge fee discounts to incentivize eligible merchants to use SFP.
The benefit for merchants is that they get to ship at the negotiated rates for UPS/FedEx/USPS that Amazon has access to. Additionally, these SFP merchants still get the Prime badge.
For Amazon, this will likely mean fewer FCs to build, and being able to save on labor, packaging, etc.
To me, it was always evident that Amazon loses quite a bit of money on shipping/handling some larger items. When an FBA merchant sells on Amazon, there's a referral fee and FBA fee. The FBA fee is often absurdly low, and might be lower than the cost to just ship from the merchant warehouse to a FC.
It seems to make intuitive sense that instead of shipping from point A (warehouse) -> point B (FC) -> point C (customer), if you can ship from point A to point C in a reasonable amount of time, it saves all parties money.
ADDENDUM:
It's also worth considering that SFP will be great for Amazon's efforts to build out its own delivery fleets. I imagine that in the future, SFP merchants will be forced/given the option to ship with Amazon's own parcel delivery service. This will be a steady base of business (extreme "dogfooding") to start off with, similar to how AWS originated with internal infrastructure for Amazon.