Always book the extra hours in the reservation so there is no dispute. Also, they can charge you a late return fee if your reservation is for 24 hours and you return it at 28 hours. (They can also charge you a late return fee if your reservation is for 12 hours and you return it after 16, but that's another story. )

Do be aware that there is no grace period on any extra daily charges, like LDW, only on the rental rate.