According to @jedberg... they don't get any discounts other than hitting those top level tiers which are discounted. They pay the same rates as us peasants for the lower levels.
And yes, I'd imagine they heavily use reserved instances as their traffic patterns would be pretty predictable by now.
I guess he could be technically right in that they might overall be so far into the "Contact Us" tier that they've just been given a steep enough discount for that part of their usage to nullify the premium on the lower tiers. Having Netflix people say they're paying the regular rates certainly is good for Amazon PR.
And yes, I'd imagine they heavily use reserved instances as their traffic patterns would be pretty predictable by now.