I guess I've never understood the appeal of making Amazon purchases with the Echo... I never cared for that feature... if I need to order something I pull out my phone and use the Amazon app... But that's just my opinion.If you have a household you can purchase from the other person's account though. My wife is in my household and she's used her account to make purchases.
Also this is not the same thing as true multiple user account support... This is just a service that supports multiple accounts... like multiple account support for music services... Which both devices support.