It's my understanding the certificates are only good for the face value "towards" (not the total) amount of the required deposit. Then the bonus is with getting the on board credit. Now, there is some conflicting information as to whether the required deposit will be discounted at time of booking, or you have to send it in after paying the full amount of the deposit and then they credit it back.
One of you, Jim or Tim, should call them and ask today and see what they say. Well, actually you both should call and see if you both get the same answer.
I found this