Just a FYI with Avis...I've recently been getting Avis promotional emails for "split my bill". See their link below. I haven't tried this yet to see how seamless it is. And it's not just for adding extra personal days to a work trip. You can personally pay for an upgraded convertible or SUV when work will only pay for the intermediate or full size car.

Split Your Rental Car Bill with the Avis App | Avis Rent a Car