Quite the opposite, people usually don't actually go out shopping just for a single trivial item, they delay the purchase until some later time when they go shopping for some important item or many trivial items at once.
Splitting the same total purchases in many separate "shoppings" is the fragmentation problem that causes much more "delivery events".
I find plenty of instances of people buying 1 thing at a store so it goes both ways. Regardless, the marginal cost of adding an extra stop to an optimized delivery route is very low compared to a shopping roundtrip for a single consumer.
people usually don't go shopping every day possibly multiple times a day. Whereas Amazon gives them the ability to order things such that a car drives up to their house and delivers stuff multiple times per day.
Splitting the same total purchases in many separate "shoppings" is the fragmentation problem that causes much more "delivery events".