Techis, I think this is a wonderful idea you have. Might I suggest two or three things:
First, if it is a charity, I would suggest doing a video interview with them, so people can see how real the charity is.
Second, sign a contract stating what you are doing with the representative of the charity, that way there is a legal basis for it. (Paperwork, Paperwork, Paperwork)
Third, once you have the contract and approval, publish any public info on the charity, so people can look it up.
Once you cover those three aspects, there is a higher probability that people won't think it's a scam, since it's all in the up and up.