This isn't the Internet where links have a bounded capacity and zero marginal cost. The trucks/planes have their schedule - more packages mean more fuel is used, not exhaustion of discrete slots. Delivery services do not make money by queuing packages.
If Amazon had asked for a single truck from UPS at 5 PM from warehouse A (the number of trucks ordered is a business decision), it can only allocate x kgs of packages or y liters of volume to that resource.
Edit: Just to clarify, the number of trucks may often be decided waaay in advance.
The cutoff windows they do give are too long to support your theory (what if demand then spikes?), unless Amazon also retracts those windows (which would also be customer hostile). I'm not saying the constraints you describe are impossible, just highly unlikely based on how every other merchant/shipper works. The tiny gain from optimally packing trucks does not seem to outweigh the additional complexity required to do so.