I've never tried it with C2, but in my experience of web-dev it's stupidly hard to get a decent direct donation to certain charities - the big ones like Cancer Research UK decline everything from SagePay to PayPal; they want you to go through their gateways.
Depending on how much you want to raise it might be worth starting your own charity and then distributing the money as you see fit:
The best advice I've got is to directly link to the organisation or JustGiving page you want to support - It's a few extra clicks and sadly that can make or break a donation... I imagine there's a good tax reason that large charitable organisations don't like 3rd party gateways but it's sure frustrating!
PayPal had a charity thing called GivingFish or something, but that was terrible IIRC, quite shady.