Unlike restaurant delivery, grocery delivery services are right now operating mostly on non-exclusive contracts with the upstream chains (because the chains are big and have a lot of negotiating power; and because the chains have a real need to partner with different delivery services in different markets, because no delivery service exists everywhere the chains are yet.) There's no real market for delivery from mom-n-pop produce markets, and so there's nobody for grocery delivery services to bully into accepting an exclusive contract with them.

If you can get the same groceries from the same store no matter which portal you order through, then the portal itself is a commodity.

On the Postmates App, yes, you definitely care about where you're getting your food. The marketplace here is between eater (buyer) and restaurant (supplier).

However, if you are a business that wants to offer delivery directly and don't want to create your own delivery network, you would use Postmates API. In this case, the business (buyer) doesn't care about each individual supplier (i.e. the fleet is a commodity).