It is an apples to oranges comparison.
Hogwash.
An immediate need like milk, bread, eggs, and so on can be quickly picked up at any grocery or convenience store. Long term meal planning can be done through a service like Amazon just as one can wait a few days for a desired book, cd, video game, even clothing can wait a couple of days, less if you have a Prime membership; and you don't the inconvenience of driving anywhere with gas at about $4 a gallon.
The issue will be prices, related cost, and quality of service.
Schwans, I apparently need to remind you both, is already a successful grocery delivery service. If anyone needs to worry about Amazon it is they.