While 15-20% isn't expected, a few dollars would be nice. At every restaurant I've worked at, a to go order is almost as much work as waiting on someone. It requires taking an order, ringing it into the computer, packing it up, putting in condiments, getting all of the sauces in ramekins, sometimes labeling, and bagging. It's not nearly as easy as say doing a to go order at Burger King (and trust me, I've worked fast food). Restaurants often aren't set up for simple To Go orders like a fast food place. Where I work, hostesses (who do To Go orders) make $1 more an hour than servers and don't get tips usually. They make less than 1/3 what the servers make.
I remember being a hostess, making $7 an hour, and putting together this huge to go order ($200 worth). I labeled every box, went above and beyond, and didn't even get $1. I didn't expect $30, but they really couldn't even leave me $5. I felt jipped.