If I'm ordering groceries on Instacart, how am I not their customer? Am I only the customer of the grocery store and delivery driver?

To be frank, I’ve never used Instacart, so anything would be (a hopefully educated) guess.

But it might be some version of Michael Seibels's classic "Who is your customer?" (https://youtu.be/C27RVio2rOs?t=10m11s)