What is a high-volume business that has mass-market appeal and can benefit from falling retail costs?
It has to be something that you'd never buy online, or that has some major advantage to physically handling before purchase... or maybe some sort of service.
Clothing, fresh food (meat, produce, etc.) prepared food (eat in and take out), and I think consumer staples (soap, toiletries, misc) that's just quicker to pick up on the way home than to order and wait for. Unless you're really well organized, you tend to buy that stuff when you've run out and you won't want to wait a few days for shipping.
It has to be something that you'd never buy online, or that has some major advantage to physically handling before purchase... or maybe some sort of service.