The problem is that the browser knows that myups.com is not ups.com. But it doesn't know that you don't have an account at myups.com and think you are logging into ups.com.

The best solution to this is using your browsers built-in password manager (or your favourite browser-integrated password manager) then your randomly-generated password for ups.com won't auto-fill for myups.com and you at least have to think about it and wonder why you need to fish the password out of the password manager.