Oddly, or at least maybe not immediately apparent, the distance usually isn’t the major factor. It’s all the other tasks in betwen that must happen.
It’s the timing when the parcel arrives at each step. Did the plane just leave or is it not leaving for another 20 hours? Did the parcel get put on the truck first and therefore will be last off at the sorting facility? Is there a ground transport segment that will cause a long delay if there is a short delay during part of the travel (ferry, train, customs, sorting, shipping container full and no replacement, etc.) The long-distance segments go without anything much occurring, it’s the ends of those long journeys where the real problems are.
It's not much different than when people travel great distances. Once you’re on the plane, things generally go well. It’s the airports, ground traffic, parking, trains, buses, security, gate assignments, etc. that screw things up.
I’m amazed with the logistics involved. If you’ve ever watched package delivery in a major city, you get an understanding of the complexity just in that final leg of the process. Drivers stop and unload a runner (or runners) with a huge cart full of packages. They can’t park, they just stop. Those runners have to deliver packages to multiple buildings just on that one block. Those buildings have dozens of addresses within them, spread out over dozens of floors. And that’s just the LAST segment of the journey for those few packages out of millions. In some sections of Manhattan, those runners go to the next block and exchange carts from the truck of a different truck. The system is mind boggling.
Rural delivery has a different set of problems. There’s so little density and frequency that there is a disproportionate amount of labor and fuel for those last few miles. None of it is simple.