The third party is working with the charity(or ngo or whatever). The charity is essentially paying them for marketing, using a huge chunk of the money people think they're giving to charity. The charity is complicit in this deception, and the third party presents themselves as volunteers "Hello, I'm with Save the Children, we do bla bla bla look at this picture of a starving child would you be interested in helping us by giving money every month to give this starving child a better life?"

They don't tell you they're paid to be there. They don't tell you the first year of payments goes directly to a private company.

I looked up Save the Children in some charity index thing a while back and it was listed as something like 94% of the money they receive goes to the stated cause which I doubt includes these marketing costs. You could say this is still worth it because they increase the amount of money the charity receives even if a lot of it goes to the company. But it doesn't seem right to me, not when they deceive people this way.