Here's how I think this works behind the scenes. First, FedEx (or whomever they use) is highly capable of handling this, as everyone has pointed out. Second, I'm sure given the size of the shipment that Apple has a FedEx shipping logistics manager assigned to them to plan this whole thing out. Their shipping costs probably depend on how much time Apple gives them to move the packages. Ideally, Apple would be delivering packages to them now, or starting within the next few days, and continuously until the 18th. FedEx would then have plenty of time to move pallets of phones to regional and local distribution centers, and they'd do it based on economics (ie. when one of their planes has plenty of extra space, they'd go ahead and put a couple pallets of phones on it). In the end they'd queue pallets of phones at the local distribution warehouses, where they'd sit until the 19th when they are loaded on trucks for final delivery to customers.
Alternatively this could all happen within the last few days before the delivery date, and it would just cost more. But I think the result is the same - get them to the local warehouses right away, then wait until the 19th to send them out on trucks.
Or, if we're REALLY lucky, maybe they'll send some out on the 18th just to spread the workload a little (I highly doubt it, but one can hope).