Comparing packages to deliveries is apples to oranges. I don't disagree that these jobs are rough but 120 deliveries with an average of 3-4 packages per delivery paints a similar picture.
Do you have any source for this? It seems like a really weird assumption to make, especially given that Amazon will already bundle multiple orders into the same package.
The 400 package number from the article is also specifically cited from this company and we have no idea whether that is a nationwide figure while the UPS comment implies that it is.
But you're still just making an assumption with 0 evidence. I would love to see a distribution of how common multi-package deliveries are, but we don't have that information, and I highly suspect that it's overwhelmingly one package per delivery.
It seems like an exceptionally bad assumption that Amazon is regularly shipping 3-4 items individually rather than bundling them up. After all, they are basically a logistics company at this point.
If we're just making assumptions here, you may as well assume that most Amazon deliveries are to apartment buildings, where the driver just leaves everything at the front desk, there are 200 packagers per delivery, and the driver is only making 2 stops per day.
Also, Amazon has a large number of different warehouses that products are shipped from, and it's quite common for different items/orders to not ship together either for that reason or because some items' availability would slow down the others.
That’s not always been my experience. Just last week, I had three separate orders, all made on the same day, arrive in three separate packages.