There is no such thing as a prepaid photopass card. You are probably talking about the photopass cd which runs about $120. This cd will include every picture that the photopass photographers take of you on your trip. You can buy the cd at the end of your trip or you can buy it once you get home.
The first time you get your picture taken by photopass they will give you a blue photopass card. Every other time you get your picture taken they will scan your card and your pictures will be sent to the same account.
Once you get home you can go to the Photopass website and open an account. You will then type in the number on your photopass card and any other photopass card you might have got throughout the trip and you will be able to view your pictures. You can then edit them and add in borders and images.
The cd will include all of your pictures including the originals and any that you modify.
For instance Photopass took 150 pictures of my family on our one week trip. After I logged onto Photopass and edited them we had 307. I ordered the cd and it arrived a few days later and all 307 pictures were on there.