The pizza guy just walked away... Why no change?

I bought a pizza the other day and it cost $20.47 for the pizza, delivery and tax. I didnt have any small bills so I gave the pizza guy a $20 and a $10... He just handed me the pizza, said "Thanks" and walked away. Why didnt he give me my change? Why would he expect me to tip that generously? That's almost a 50% tip, for Pete's sake! Why don't pizza guys give any change back? Are they not supposed to or something? I feel totally ripped off.