Do they charge your before the order actually ships? I don't think they do.

Charging your credit card might not coincide with their definition of revenue recognition.

What if a customer cancel the shipment?

The point is that they don't hold the inventory.

They buy stuff on net-30 terms, and ship stuff to you the day it arrives. WalMart does similar things for many products -- they make vendors carry inventory until it arrives at a DC.

Unless they've changed something recently, they don't. I haven't paid enough attention lately to be sure.