These services charge more for the items purchased than what you would expect if you were to purchase them at the store yourself. The $4 per delivery is simply a facade.
Kind of a shame Google won't charge more for the service. I do not want it to go away. I would pay $30/hr and up if they also did perishables instead of having to figure out how much more random amount I'm paying instacart or other services. I would even use it to replace my Amazon purchases to an extent...
Instacart: https://www.instacart.com/faq#inventory-and-prices-q2
Yes, Instacart prices are our own and vary from the store’s price.
Postmates https://postmates.com/help
Additionally, a 9% service fee is applied to the purchase price of your items.