I don't agree with that sentiment. I think your pattern detector is only taking in account on how grocery shopping is currently done. When you introduce something new into the system, it may not adhere to the same rules. There are many cases when convenience will trump price. Say you need to buy items from 10 different stores. I would rather use a service that goes to 10 different stores for me, than have myself drive to each store to hunt down each item.
When I use amazon prime, I'm very aware that the prices are sometimes more expensive than a generic brand, but I am willing to pay the small price difference for the 2-day shipping convenience. A study was released on amazon prime shoppers that showed prime customers are less affected by price. The convenience of getting something with 2-day shipping, but with a higher price trumped a cheaper item but longer delivery times. People don't always go for the lowest cost item.