When I was a server at a sit-down restaurant many years ago, carryout meals meant I had to answer the phone, answer questions from the customer, take the order, enter it into the computer, time the order for the anticipated pick up time, follow up with the kitchen so they knew it was carryout and packed sauces on the side etc if needed, gather the containers, pack up the food, add plasticware, be available when the customer arrived for pick up, ring up the bill and get the credit card signed or change returned.
While I wouldn't have expected a 20% tip, it can be a lot of work. 10% or a few dollars would have been appreciated.