I always tip at least $4.
That's interesting. I don't use any of the delivery services, only the ones offered by the restaurant (usually pizza). Last I knew their delivery fees do not all go to the driver. Not even half when I last asked. The rest is probably used to cover insurance, etc. for operating delivery.
When I did pizza delivery no place charged a delivery fee. Most pizza chains gave 50-75 cents to the driver per delivery. My local Papa Johns paid 5.5% of the sale at the time (they stopped this years ago). Great if you took a $100 order, sucked if you took a $6 to the furthest delivery location and got no tip.