In car rental, fuel purchase option is an optional service offered by many suppliers whose default fuel policy is Full to Full. Full to Full fuel policy means that the renter will receive the rental vehicle with a full tank and they must also return it with a full tank.
By choosing the fuel purchase option and paying an extra fee for it, the fuel policy is changed to Full to Empty. This means that the renter will receive the rental vehicle with a full tank, but they can return it with an empty tank.
For example, Budget and Hertz are one of such suppliers that offer this service for their customers.