This type of thread seems to re-appear every two years.

Income has nothing to do with tipping. Waiters at 5 star restaurants who pull down 6 figures expect to be tipped. If someone making minimum wage has dinner at a diner, they are expected to tip.

Tipping is based on service. I agree with Rick: $1/bag. If you don't want to pay then carry your own bags. If the driver is there waiting for you and you don't have to call/wait, that deserves something.

Plan ahead and stock up on singles the days before your tip. Or at least carry some cash like a normal person.

Just don't try to rationalize.