Real-brisker: You misunderstood the delivery “boy” (referred to in your first comment). You misinterpreted the inquiry of the delivery “boy” when he asked about change – he was actually being courteous to you, he was not asking for a tip. A 15% tip on a bill of $26.25 would be about $3.90, and you were owed (assuming no tip) only $3.75 in change. The delivery “boy” reasonably (and perhaps with chesed) assumed you were like most people and did not expect change, but he had the courtesy to offer you an opportunity to make clear your intention not to tip him. Tips may be optional according to Webster, but Webster is no Rashi. Many, if not most, delivery people work only for tips, and you should recognize that before you avail yourself of their services. Your objection that the restaurant advertised “free” delivery is a phoney excuse for your discourtesy. And if you are going hungry, as you stated, there are many charities that can help you, and I suppose you don’t have to tip them if they bring the meal to your home.
If you habitually do not tip delivery personnel, and if you ever serve delivered food to guests, perhaps you should warn them that their food may have been adulterated by an untipped delivery man or woman.