Amazon's logistics and approach discourage bundling, while Costco's in-store approach incentivizes it.

So it's more like 100 people drive to Costco, and they each buy 20 items and drive home. Or one Amazon delivery truck makes 1000 separate deliveries over the span of a week, because those 100 people made 10 different orders each, only ordering 2 items at a time. (I've even run into the situation where two separate Amazon orders made on the same day [because I forgot something in the first order] will arrive two days later, on two separate trucks, at two different times of the day.)

This part bears repeating in a different way: if I go to Costco and get 20 items, I drive there and back once, on one day. If I order like people typically do on Amazon for those same items, I have a truck/van visiting my address 5-10 times on a bunch of days over the span of two weeks.