Try returning the car during opening hours and have the car inspected by a competent employee. The condition of the vehicle should be confirmed in writing and signed by a representative of the car rental. In Iceland many companies do not issue a statement of the car being in good condition unless after it has been cleaned. It is advised that you wait until the car has been cleaned so you can get a statement of the condition of the car.
If you are returning the car outside opening hours, you should park in the designated area. It is advised that you take dated pictures, as well as pictures of the mileage of the vehicle, as confirmation that it was returned in good condition.
Cars returned outside working hours are inspected for damage on the following day and therefore the excess can still be charged to a consumer´s credit card.