Tipping pizza delivery guy

It was always confusing how much to tip and whether to tip, when the store already adds delivery charge on online order. I called the pizza hut and they told it as store charge. Some other time I tried to order the pizza on phone order, they told it was 10, which was 6.99 with online coupon. This looks tricky and confusing to me.

Google shows tipthepizzaguy website.
"The common courtesy is 15% for normal service. On small orders, when 15% is less than $2, the proper amount is the $2 minimum standard. It should be a $2 minimum to have someone come to your house. 20% or more if the service is excellent. 10% or less for poor service. Nothing for very bad service. If the order is $50 or more, at least 10%. 1 or 2$ more for long distance and bad weather."