Joseph (in the guise of real-brisker, Trying my best, and Cedarhurst):
Saying “excuse me” or “sorry” if you smash into someone and knock them down is also totally optional, but in U.S. culture, one who doesn’t apologize is seen as a total boor. Tipping for certain services is also a part of U.S. culture, to the point that a non-tipper is seen in the same light. To NOT tip in this culture is to communicate that you are either highly dissatisfied with the service provided, or simply rude.
A Yid in golus should not do something that would cause others to view him as a total boor. If for nothing else, do it for that reason.
All that said, seriously, if you didn’t tip the poor deliveryman, I recommend not getting delivery from the same place again.
PS – In many places, the tips ARE included in the price if the party size is large (look at the small print on some menus).