There is no expectation of a tip if you do carryout. They give the option, but don't feel obligated.
I never do it on a PIzza pickup, because all they did was slap it into a box, big deal. I have been known to sometimes give a few bucks ($3-5 depending on check) if its a restaurant that goes to a lot of trouble of packaging up takeout of a variety of plated items, just to be nice. But even then there is no way I'm doing the usual 18-20%- that's only if you wait on me throughout an entire meal service, bringing drinks throughout, etc.