Late night school dinner

Another late night dinner before my night class.  I got this falafel sandwich combo from a local restaurant near work.  I guess it's because I'm not a dine-in customer.  The hostess and waitress are a little snappy.  They are losing tip.  I don't tip for take out and I don't think I should.  Asked several people and they don't tip at take out either.  Tip is for service right?  I mean this food is like $8.  And they really didn't do much.  The food is really delicious, enjoyed a lot.  But not sure if I should go back to the same restaurant :(