I am having the same issue as well. I would buy "RuneScape 3 [Instant Access]" membership through Amazon for one of my alts but ever since upgrading to Jagex Account, I can't purchase it anymore.
To add more context: When purchasing membership through Amazon, you need to add the character that the membership will go to. Amazon identifies your character as a "vendor account." (I don't know why but this is what they call it). As Thorbif mentioned, you get a pop-up with a login screen so you can point the purchase to your character. Before, it was a simple add your email/password, hit "login" and you were good to go. Now with Jagex accounts, you cannot use the old login info anymore. Just to confirm, I've tried my old login info. Nothing. I've tried the new Jagex account login. Nothing. I've tried creating a new vendor account with the new Jagex account info. Nothing.
The fix to this issue would be to either update the linking through Amazon or to supply a key code so that people can redeem the membership that way.
17-Jun-2023 03:11:42