Each reward tier provides a dollar figure of how much you should contribute in order to receive an additional item.
For example, the "Pledge $15 or more" tier (which includes one deck of BOOM GOES THE DYNAMITE) states that you can "Add $15 for each additional deck." So, if you want two decks, you will pledge $30 in that reward tier.
At the end of the campaign, we will reach out to each backer to confirm his/her rewards for each pledge. So, in our example, if you pledged $30 toward the "Pledge $15 or more" reward, you will have a chance to confirm after the campaign ends that you would like two decks of BOOM GOES THE DYNAMITE for your $30 pledge.