Standard tip is 15% percent for decent service, 20% for really good, just like in any restaurant.
Silverseven, I think what she is trying to say is that when you get a pizza delivered, it already usually has about 10% for a delivery charge tacked onto the bill. I know that some of this money goes to the driver, so what the OP is trying to figure out is, how much more over that the tip that is already included in the price should she give them?
It's hard to say what 15% of the cost of the order is when there is already money tacked on to the original order for the driver.
As for the original question, I think you did fine. I usually tip between 3 and 4 dollars each time.