Interesting that they’ve always tried to charge you for the upgrade - I’ve always gotten it for free without having to ask (though I use Avis almost entirely and can’t remember the last time I used Enterprise). They just tell me “Sorry we were out of this car, here’s an upgraded one”. Seems weird that they should charge you more for not having the car you booked. Granted, they never dump me into sports cars when it happens (it’s almost always the very next tier of car available).