They do not send you Photopass cards. Like bethymouse said, you get them from the photopass photographer.
When you get your PhotoPass card, make sure you either take a picture of it or write down the number clearly and legibly and put it away so in case you loose your card. If not and you loose the card, then you will not have access to the picture you had taken.
If you plan on having PhotoPass photagraphers take lots of pictures then get the prepaid CD and make sure you stop at every photographer and get pics. With the CD you get access to the raw picture which you can later have developed at anyplace you desire for far less money. I think the current cost for prepay is $149.00 and to purchase after arrival it is $199.00. The cost for one 5" x 7" in the parks is at least $10 or more. With the CD you can put multiple PhotoPass cards on it. This means if everyone in your family has a card then when you get home you go onto the website, enter all the numbers and you have all the pics.