Some years ago, I met a guy at a bar in New York who said that he was Donald Trump's personal courier. He rode a bike around the city, and he'd deliver things to Trump in person, who would always give him at least $100 cash on the spot. I didn't believe the guy's story, and it offended him. Maybe he was telling the truth.
IIRC the climax of the merger deal described in /Barbarians at the Gate/ essentially comes down to a bike courier race between various offices in Manhattan as last-minute bid adjustments got ferried about. Makes me wonder what the value of the truly fastest bike courier in New York would be to some large investment bank/PE firm/whatever. I imagine they are massively underpaid compared to the value they provide.
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.