It'd seem fairly trivial for, e.g. Amazon (as in this case) to ask for your Amazon password if you go over a certain dollar-spend - or even weigh the risk of va

It'd seem fairly trivial for, e.g. Amazon (as in this case) to ask for your Amazon password if you go over a certain dollar-spend - or even weigh the risk of various product categories (e.g., you commonly overnight items from household goods, so we'll trust your thumbprint; but you've never ordered 5 TVs before, you're gonna need your password for that one).