It wouldn't take much. Amazon already has the warehouse space. Adding an outside covered area where they could brink your loads out to your truck is pretty cheap. And if you have 40 routes, and they deliver to your location for sorting, etc, you are going to need a 100 x 150 warehouse, or schedule drivers to come in an order you set up.
I worked years ago at a warehouse where drivers lined up and pulled in to get their load in the order they got in line. Must have been 150 trucks. Some people liked to get there early, others didn't mind a later start. They could handle maybe 25 trucks inside at a time, so the line was nearly constantly moving. They had an employee watch ahead to see what order the trucks were coming in so orders would be set up as you pulled in. It was a very efficient use of the space they had.
If Amazon palletizes and wraps the load, it would be pretty simple. When I was at HD, loading was a cluster**** with unloading incoming packages, sorting, moving, loading, and scanning all trying to be done simultaneously. I'd expect almost all Amazon shipments to be automatically sorted so each route could be moved in bulk to your trucks. Either those cheap plastic pallets, or cardboard 'sliders' could be utilized to make the process more organized.
Basically think that sorting, where, when, and who is a minor issue and easy to overcome. I would assume/hope that they have accounted for any of those costs in their profit estimates.