The 3rd party seller opts into this bundling as a benefit to offer their customers, though, right? No one is forcing seller X to use FBA or Amazon’s platform at all; they’re doing it because customers prefer it now that they see what’s possible.

To me (in the US), Prime was magical when it first rolled out. It lowered the friction and raised my expectations for online commerce in general. When other players followed suit, my life got even better. Now, someone charging $20 to get me a package “maybe next week; you’ll get it when you get it” is wildly uncompetitive rather than being the accepted standard.