I'm trying to change the collection of a vehicle by a few hours but when I try to edit the reservation (modify, keep my rate) the system says no cars are available. I'll be arriving later than expected - is there anything I can do other than adding a note to the reservation?

Incidentally that reservation is for a week. I need to return the car on the 6th day. I've tried to amend the reservation and I have the same problem as above. Will it be a problem just to return the car a day earlier, or will there be any re-pricing?