Maybe your account name, but why would it transmit a password? It knows that it's receiving the order from a Kindle device you have physical access to. Why would it transmit a password, especially in the clear? Doesn't make sense. And even if it did, all the bad guy would get access to is the Amazon account, which never shows full credit card numbers once they're entered anyway.

ETA: I see boberonicus posted essentially the same thing. Sorry