A vendor would probably use a person's previous purchases and purchasing trends of similar people to figure out what to send. Make returns very easy, should just be a matter of resealing a package , putting an enclosed sticker on it and dropping it off somewhere. Perhaps notify person ahead of time that a new order is being put together for them, allowing them to cancel any time before it is delivered.
Imagine getting deliveries that you didn't specifically order, so you don't know what's inside. It would be like getting gifts from yourself with the added element of surprise.