Ko-fi don't take a fee at all.
We don't get involved in the payment at all. We generate a link so you pay the creator directly to their PayPal, on their PayPal page. Paypal normally take a small standard fee from that transaction, depending on the creator's PayPal account set up. If the creator has registered for micro-payments with PayPal this can be 5%.
Otherwise, it's typically and industry standard 20c +2.9%
We get a notification from PayPal that a transaction has taken place so we can update the creator's page.
Hope that helps!