These guys expect you to predict the future. Its not as simple as some amount per day till whenever you return. They offer sort of a low rate, with many hooks in there if anything changes. A lot of time I fly out, not knowing exactly when I am going to return.
After about 5-7 days, I know, but when I leave the car there and book online, its required by software to enter a return date, if you are long or short, they gouge either way and jack the rate sky high. They have adopted the airlines model on flights with 1-2 days advance booking.