That's the same line of reasoning many of the "on the ground" fundraising companies use (i.e., the ones that send people to street corners to solicit donations... many of them do not actually work for the charities they're soliciting for).

Just because you have (ostensibly) a net positive impact on fundraising for a good cause does not remove your obligation to behave ethically and divulge pertinent information. 9% is a lot of money, and this needs to be a lot more visible than it is now.

Think critically about this: if your rates are widely known across the website, would this positively or negatively impact donation rates? By how much? The next pertinent question is: is your business model built fundamentally upon user ignorance of your cut?