From their Google Doc transparency file.

"100% of every donation to patients on Watsi directly funds medical treatment - our overhead costs are covered by optional donor tips, cause marketing partnerships, and private donors, foundations and corporations."

https://docs.google.com/spreadsheets/d/1KQ2uWkJf5-5f34uttGmA...

Thanks, Josh! Our operational costs are covered by foundations, philanthropists, and donors who leave an optional tip during checkout. More in our FAQ: watsi.org/faq

Donors have an option of "tipping" Watsi apart from the donating to the patient's treatment. But I'm not sure if this alone covers their operating costs.