Zero, because people who want to buy from Amazon will just try again in a few minutes/hours when it is back up.
There will almost certainly be some number of people who would have stopped by Amazon right now and made some impulse purchases. At the scale Amazon operates, the increase in inconvenience to push off the marginal purchase as a function of inconvenience is almost certainly miniscule (See frequent reports on how milliseconds of page load time affect the likelihood of purchase)
(Surely there's some loss from being down, but it's not a simple loss = current order rate * downtime argument).