Hey, I think I may have figured it out by reading some other related posts. The photopass card that you get in the Mickey Mail is really there so people are aware of this option. By registering the card, you may receive some discounts on printed pictures (or not, there is no guarantee), but the CD will still cost you $169 if you order it after you arrive.
Alternatively, you can pre-purchase the CD for $119 thus savings yourself $50. You can use any photopass card for this (any you get in the park, or if you end up taking the registered card in your Mickey Mail).
The downside for registering the card is that you have a shorter time frame to "claim" your pictures. The countdown will start as soon as the first picture is taken at the park. On the other hand, if you don't register the card, you will have 30 days after your last photo to register, and then 30 days to edit (essentially giving you about 60 days to receive your photos).
HTH!