just to clarify, there are many ways to use photopass.
Photopass itself is just a system where everytime you see an official photopass photographer they will ask if you already have a photopass card and if not give you one. That card has a number on it which they scan. The photos they then take for you are linked to that number. You can re-use that card for your entire length of your stay or collect more cards if you accidently leave one in your room one day.
Before your trip, you CAN (but dont have to!) buy a pre-paid CD. IF you do this, then when you get home again you can go onto the photopass website and have all (or just the best) of the photos added to the CD and sent to you.
IF you choose not to do the pre-order, you can still use photopass in other ways. Whilst on your vacation I think you can go to several locations and get pictures printed from your photopass card - at a cost obviously! Or you can do that when you get home.
OR you can buy the CD after your vacation but not at the discounted pre-order rate.
Once that makes sense, you then might want to investigate photopass+. But I dont know much about that lol!