Unlike other services, Ko-fi don't take any fees from the transactions between supporters and creators. We want the creator to receive the maximum amount of the donation from the supporter and we want creators to get their donation funds instantly.
The payment processor i.e. PayPal does take their normal transaction fees which will vary based on the type of PayPal account, the currencies used and if the creator has enabled micropayments.
When a supporter chooses to donate to a creator, Ko-fi generates a link to that creators PayPal page. The transaction takes place directly on PayPal. We get a notification from PayPal that a transaction has taken place so we can update the creator's page, but we are not involved in processing the payment at all.