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...

Some do:

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.