There is an screen for online award redemption (step 4) where you are asked to choose the method of payment if you have a Visa certificate. You can either attach the Visa certificate or order another certificate (with the required point deduction). The information is not hidden, and you should be able to see it on the browser. I think it is fairly obvious unless you are rushing to confirm the reservation and do not bother to read the information written. If that is the case, Marriott is not at fault.