Yes! Here's my current ad hoc solution: select the base reward that you want, and then add another $20 for each copy of the book you want included in that order. When delivery time comes, I will combine all your rewards into one package and contact you with any additional cost for shipping (adding more than one additional book will probably bump shipping costs up another $10 USD or so to most regions). We'll settle up via paypal when I know exactly how much it will cost.
For example: If you select the $20 level and pledge $40, I will combine 2 books into your shipment (covered by the $20 international shipping price). If you want the $50 level and have a friend who just wants the book, select the $50 reward and pledge $70, and I'll combine the two books with the shirt, etc.
I'm not interested in doing a full add-on thing for this project, and this seems manageable to me. But If there's a better way to save some money for you international rockstars, I'm all ears!