I've had this happen a handful of times, starting at least 5 years ago. I haven't had it happen in >2 years since I now just book extra days if there's any chance I'll keep the car longer (I've never been hit with a charge to return early). I thought it was $10 if you changed prior to the original return day or time (can't remember which) and about $25 if you did it after that point. Unfortunately, in your situation, the only options are to change the return time before driving off the lot, or, paying the fee.