This is not unbelievable. I don't even live in a coastal city and I still know a handful of active couriers. They're faster than drivers in congested areas and pretty much get a pass on all traffic control/regulation if they don't truly endanger pedestrians. Not to mention that they don't need parking, can carry their vehicles up stairs and into buildings etc.

Plenty of people know these guys exist and having someone known to you and reliable on speed dial is worthwhile. The $100 also makes sense because it isn't just a tip, it is the 'retainer' to make sure the calls get maximum priority.