When reserving the car, consider arranging your return time for the same time of day as your pickup time rather than an hour or two before your flight time.
If, for some reason, your flight is delayed (which happens more frequently later in the day) and you have to extend the use of your car beyond the time you prearranged for return, there will be additional hourly charges and possibly an additional day charged if you go beyond that first extra hour.
Example: You pick up your car on Day 1 @9pm and plan to return on Day 4 @2pm. You are charged for a car per 24 hour day so you have essentially paid for it until 9pm of the return day. BUT, if you tell them in advance that you are returning it earlier, at 2pm, your contract then ends at that earlier time without credit for unused hours of that last day. Then, if your flight is delayed and you wish to keep your car more than an hour past your pre-arranged return time (2pm), you may be charged for an additional day!
So, in these days of increased flight delays, why not provide yourself some "free insurance" and just pre-arrange your return for the same time of day as your pick-up time even though you plan to leave hours earlier. You shouldn't be charged for early return of the car (but don't expect a credit either) and you have the peace of mind that if your flight is delayed by several hours, you still have your wheels and don't have to spend that additional time sitting in the terminal.