Hi Brittany!
Disney's PhotoPass is a great service to use, and yes, there is no charge to use the service. There are two ways to get a PhotoPass card:
If you've secured a Magic Your Way package, you'll usually get a Disney PhotoPass card in the mail. Once you get it, you have to go to www.disneyphotopass.com and register the card. Or...
If you take a photo with any one of the Disney photographers, they will give you a PhotoPass card if you don't already have one. What they do is take your photo, then bar-code scan your card. That ties the pictures you just took to the unique card number on the Photo Pass.
You can then go to any Kodak Photo Pass Center at the Parks to view your photos. Or, you can just wait till you get home, register with the Photo Pass card, and view your pictures post-trip.
There is no extra charge to take pictures with characters at the meet and greets. If pictures are taken
at the character meals, however, they may offer photo packages at a cost. If, however, you decide to get photos either at the Kodak Center or at home, there is a cost for those...
Here is a good link to get more details on Disney's Photo Pass (click here).
In terms of Park Hopper, you can add the Park Hopper option at any time, even after you've gotten to the Parks or resort if needed.
Hope this answers all your questions. Have a terrific time!