I expect the funding model for self driving taxis will be like validated parking. You ride free (or reduced) if you spend $x at whatever business, shopping mall, etc.
Ok, so it’s an interesting idea to use personal transit, but it could be done already without self driving vehicles, it might be economical. But it would be pretty niche.
Honestly, I'm surprised it's not something that Google have already built heavily into Maps. People often search for what they want (e.g. burger, pizza) - this would be an obvious time to offer deals or discounts to drive customers to particular businesses. You then expand to compete with particular retailers - someone searches for Nike specifically, they get offers from the nearby Adidas store.
This is not really any different to what happens with Google searches, where advertisers can buy keywords related to their competitors.