"At Watsi, 100% of your donations directly fund medical treatments. Watsi.org is separately funded. They pay all their operational costs from their own funding, and none from your donations. They even eat the credit card processing fees."
It doesn't sound (as) cool me if the founders can only do this until they go broke. It looks like they do accept direct donations to fund their operating costs, but it seems to me that the demand (if that's the right word for the desire to donate) might not increase at the same rate as the demand to fund treatments (and therefore costs for credit card fees, hosting, staff) increases. Better to take a percentage and be transparent about what you use it for.