If you have the Avis app, you can try to modify an open rental and it will show you the new cost. I've never seen it denied, but the cost is usually much higher, unless you're on a corporate account with good one-way rates. If you do change it via the app, TAKE A SCREENSHOT of both the estimate, plus the confirmation with the new price. I had it once where the local station added $1500 a week after the return was made. With the screenshots, Avis refunded me the $1500.