Yes you are, a courtesy tip. Not necessarily a percentage but kinda judged how difficult your order was to prepare.
Ie: my mom orders soups "to go" at restaurants all the time, soup is relatively easy, you just pour it into a bowl. so she might leave a dollar.
I like Fajitas from "On The Border" and I know from experience that it is a pain in the *** to gather everything for the fajitas, so I tip more like 5 dollars maximum. Unless something about the service was outstanding.
Be good to a waiter and they will be good to you in your future visits, and yes we do remember you if you tip well or badly.
To-go attendants don't expect to make their money in tips, they get a higher hourly pay for that. But a courtesy tip is always very much appreciated.
Also differentiate between a "carry out business" and a restaurant with carry out service. Restaurants are not set up for easy carry out service, and thus more work goes into preparation.
So the restaurant carry-out service is a "COURTESY" for guests who would not like to sit down and eat at the restaurant. So again, you are not tipping for the service, you are tipping for the courtesy, as you might a bellhop or vale.