We've built this campaign around "Add-ons." That means that each tier acts like a "base" or a "starting point" for your pledge. Simply select the version of Cross Symphonic you want and then add items on from there. You do this by adding the cost of each add-on item to your pledge - you don't have to change tiers!
There are detailed instructions at the bottom of the campaign homepage that will walk you through the add-on process.