1. Go to the right hand corner and click "manage."
2. Click on "change your pledge."
3. Add the add-on cost to the current total and continue.
4. At the end of the campaign you will get a survey asking for your address & reward info. Include the add-ons.
A mixture of both of these decks is enough for 7 people to play. I recommend keeping the game to 6 players and below because it can take a lot longer for large numbers of people to play. 6 people can play for around 30-60 minutes. 8 people can play for 50-70 minutes.