@shmoel: Many of the delivery “boys” are men trying to support their families. It’s not mandatory to give a tip, but it sure is nice. Depending on how big your order is. If your order is for a major Yom Tov, tip a bit more. If you get deliveries from the same store most of the time give what you feel you are comfortable giving.
Please keep in mind that if you in a house/building with no elevator, you should definately give something.
I usually tip a one or two dollars. Also keep in mind that tip money is not taxable, so whatever tips the delivery men make is full-take home pay.