Another option to have a cargo bike pay for itself is to not earn any money on it at all, but rather displace other expenses.
If you can sell/stop using/stop insuring/stop licensing/don't buy a car, then it may well be worth a lot of savings. It might take a big commitment, but going from a 2-car family to a 1-car family, or a 1-car family to ZERO-car family may well pay for your cargo bike without resorting to building a commercial service.
I can often carry more on my bike than is convenient in many cars.
