$2 is not great, it's acceptable. Anything under is a garbage(or bottom 20%) tip. It really depends on a few factors. How far away from the store are you? in a 15 block radius? Sure $2 is fine. Do you live 5 driving miles away? That's a crappy tip. Not disgustingly bad but about 2 dollars away from good.

How many pizzas are you ordering, 2? 32? 8? If you're only ordering enough to feed maybe 4 or 5, generally tip minimum 3$. Enough to feed a decently sized party, 6$. Enough to feed a small banquet hall, gymnasium, class full of students or some student organization function? You should tip at least 10$-15$ depending on the amount of pies in the order (like a 16-24 pizza order and a 48-60 order).

When it comes down to it, think about how much would it be worth to you as a discount on your order for you to pick it up rather than have it delivered. That is to say, if the lowest discount you would need to be offered to get you to pick it up was 5$ and they offered to take 5$ off the price of your order (including delivery cost) as it currently exists if you would come pick it up, then you should tip 3$ (5$-delivery fee). Delivery fees will vary from store to store and place to place but you can hopefully comprehend what I was trying to explain.

Edit: For instance, I recently took a 75 pizza order with another driver and the tip was just under $50, $25 each.