If you're at the $28 and up rewards levels you can purchase “Add-Ons”. There are 2 ways to add extra t-shirts and copies of the books (Mae and Oddities & Apocrypha) to current rewards. Each Add-On item is $25 either way.
1. For each extra item add $25 to your current pledge. After the Mae Kickstarter campaign is over, you'll receive a survey giving you the option to apply your unused credit to purchase more t-shirts and books beyond those included with your standard rewards level.
2. When you receive your backer survey after the Mae Kickstarter campaign is complete, you can add funds then to purchase Add-Ons.