It's a location thing. A number of stores have tried this by me, but only one has really pulled it off. People often use it for their big family orders that are well upwards of $100. In general, the place also has better prices than other stores. There are repeat customers, but shopping online certainly doesn't account for the majority of their grocery store runs. That said, the place constantly has a line of cars with huge orders being loaded into their trunks. Never small orders.

Make sure you're in an area that's largely populated by families, and make sure your prices are already good.