Tell him you have them and will get them now.
"Of course," you lie, "let me just go get those out of my car." You give the man a smile and walk towards your car, but what will you do now? Without those breadsticks, this order is incomplete; there goes your tip! Maybe you can find some before he notices?
Give him an empty box from your car and bluff until you get a tip. Jump in and drive off! Try to obtain some breadsticks and make it back before he notices.