I'm not surprised that they are giving up on Amazon Pay, as that never seemed to get much traction or customer recognition.
However, I am very surprised that they are abandoning their own first-party payments to Stripe. Don't you all already have the very best rates possible, directly negotiated by every bank? In other countries Amazon has even threatened to stop accepting cards because they couldn't negotiate the fees they wanted, and they are about the only merchant big enough to dare doing that.
I'm sure they won't be paying Stripe the standard 2.9%, but still--what value does Amazon get out of this? Stripe is supposed to make payments easy from both a coding and business perspective for developers. Everything Stripe does, from card acceptance to fraud handling to UX to ACH payouts Amazon already has working at large scale.