Yes, exactly. When you get home you will log in to the photopass website, and enter your photopass card numbers. When you go to your cart, you'll see you have a credit already in it.
Just remember that if you are going with another family that may want a photopass too, you can combine them on the same order. There is no limit to the amount of pics for one photo pass. We went with 3 other families and all split the cost of one photopass. 3 cds in total pictures for the one photopass price. Hope that didn't confuse you more...I knew what I was trying to say before I typed it!
On another note...the photopasses from the waterparks are the best. They are small and have a rubberband so it will stay on your wrist. Also take a picture of the back of your photopass just incase you lose it. Disney can pull up the pics on that card with the number alone. Also ask about "magic shots" in all the parks...very cool!