In short, don't.
Today, the company I work for had two of it's clients apps rejected because they stated within the app that a portion of the profits made will be donated to charity.
Apple's App Store Review Guidelines (requires a developer account login) state the two following points:
21. Charities and contributions
21.1 Apps that include the ability to make donations to recognized charitable organizations must be free
21.2 The collection of donations must be done via a web site in Safari or an SMS
The first point prevents any app that makes donations to charity from being a paid app. Even if the application doesn't provide an actual means of donating as a feature, the act of donating profits from the sale price counts as an ability to make a donation.
Why would Apple do this, you ask? Are Apple evil? Do they hate charity? No, at least, I don't think so.
There are some people out there who (for whatever reason) don't like to or just don't want to donate to charity. Apple is protecting their right to choose to donate to a given charity while also protecting the rights of people who do want to donate (nothing stops them from donating outside of Apple, for instance). Maybe these people don't agree with the charity's ideals or maybe they have some other reason. However, just because someone doesn't want to donate to a charity doesn't mean they shouldn't be able to use your app. The choice of using your app and making a donation shouldn't be mutually exclusive, and I agree with that.
There is also the question of legal liability - however, I can't think of a case where that would apply without it also applying to Apple paying developers who don't make charitable donations... so it seems less likely that this is a reason.
The solution, obviously, is to comply with the guidelines and make the app free while providing a website or SMS based donation feature. However, most companies simply remove the copy that says the profits will be donated - which allows them past the app store approval, but puts them in my "you're a shady company and I don't like you" book...