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.
Google Shopping Express will also use your own Costco membership to make purchases (and I believe they do similar for other store discounts/memberships). I just got a delivery of Frozen on bluray at the Costco sale price of $15.99 and I get my 2% back too ;)
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...