I'm not quite sure tbh. When I used travelking's service the hotel clerks told me that the method that was used for payment was classified in their system as *hidden* indicating to me at least that credit cards were not involved.
That is something new for me to hear, and would rather be surprised if it is the case. This is however a different agent from what I infer, so it could always be credit cards on this one. Maybe the person who tries it out can confirm from his booking.