If you carry the sled level all the way to your house, you're not moving it against gravity.
Although you're huffing and puffing and sweating by the time you reach your house, still, according to the Physics definition of work, you have done NO work on the sled all the way home.
Due that the velocity is constant that means that friction force is equal to the force exert by you, otherwise the refrigerator will accelerate or decelerate and in both cases velocity will not be constant. So then the friction force between refrigerator and floor is 150 Newtons.
