App that allows you to have the passenger seat for yourself

I don't know how this would work, and it could be more of a website than an app. But I would love to have an app where I can pay a bit less than having another passenger by paying for the passenger seat. I would get a slightly more private space and I would be helping paying for the carpool, that way everyone else would pay a bit less.<br><br>Problem is, how can we make sure that it is the passenger seat that is taken? Would have to book?<br><br>Any feedback would be appreciated.