Prompt service ensures a good tip. The driver will get paid either way, and he'll get tipped either way. However, since the tip reflects the quality of the service, the amount cannot be determined ahead of time. I'd never stiff the driver though, regardless of how bad the service is.
I don’t think your assumption holds for repeat interactions. Drivers want to get consistently tipped, so they do have an incentive for treating people who already tipped them well, namely insuring that those people will remain customers.
> I don’t think your assumption holds for repeat interactions.
I would agree with that. However, I don't order enough pizza to be a "known" customer. If I had a rapport with a particular service vendor, I wouldn't mind tipping up front, provided that they have demonstrated a consistently high level of service prior.
