This seems like a great idea but absolutely awful in execution, at least for the airport option. The T&Cs give you a 15 minute window to get your car, otherwise you have to go to the rental agency like normal. Trying to time to a 15 minute window when coming in on an aircraft seems absurd and puts people in a situation where they either put a reasonable arrival time (say 30 minutes after scheduled arrival if they have luggage) and then have to go to the rental counter if they are delayed even a little bit, or they put extra time (say, 60 minutes after scheduled arrival) and then the plane arrives 30 minutes early and they're twiddling their thumbs for an hour.

A better execution (but would likely require more labor to pull off as they can't schedule pickups as tightly) would be notifying Avis when you have arrived (app could even be programmed to send a push notification upon arrival) and the user then selecting a pickup time within the next hour or so.

PS: $5 extra for arriving in the last 5 minutes of that window (outside of a "verified flight delay" seems incredibly cheap for a service being promoted as "First"