The website has to be able to inform you about what exactly you are opting in to (like saving your shopping cart, and/or who they will be sharing the respective information with). This can’t be covered by a predefined set of options.
Browser-level permissions are about what the browser is sharing with the website, which is a different thing. For one, the browser sharing information with the website isn’t a blanket permission legally for the website to do anything with that information it likes.
You can login and buy things.
But how do you choose whether the shop can kleep track of what you have bought to suggest rebuying or for you to keep a shoopping list. Requestion those is more than login.