If I try shipping the same package with the same weight through the Postal Service it costs me a lot more money.
They don't lose $10,000 delivering the 200 packages in the bundle for Amazon. They lose it shipping my package at a "bargain basement" rate that is subsidized by Amazon.
If I have a delivery business that costs $100,000 per year to operate and I deliver 10,000 packages, then the average cost is $10 per package. But if 80% of my cost is from operating a building and buying delivery vehicles and 20% of the cost is from paying staff and fueling the vehicles for delivery, then I can still make money by delivering a bunch of packages for $5. That’s because the cost of the building and buying the vehicles is a FIXED cost that I have already paid regardless of how many packages I deliver.