I suspect the wield this lack of choice as a weapon against the carriers. They go to each carrier and say, "that a healthy looking Amazon contract you've got th

I suspect the wield this lack of choice as a weapon against the carriers. They go to each carrier and say, "that a healthy looking Amazon contract you've got there. Shame if we suddenly stopped sending packages your way", then squeeze them for price. Harder to do that if Amazon let their customer pick the carrier. I've certainly been part of something very similar, where a company has multiple suppliers providing the same service. Near contract renewal time we would move our volume on their competitors to remind them that they needed us more than we needed them. It was a very effective way to get them to keep lowering their prices as the years went on. There is also probably also a logistics piece as well. Amazon will be shipping products to carriers last mile hubs or similar themselves, rather than having a carrier handle the entire distance. Which makes it cheaper for Amazon. So carrier will also be picked based on which hub is nearest to one of Amazons warehouses, how full the Amazon truck going there is, which warehouse has the product etc