I've also been working with a couple of art director friends to create t-shirts and other items. The vendor charges me a flat fee, I upload the image, you place the order and are charged by the vendor, and a portion of the costs come to me - again to be used primarily for gift purposes.
I will be bringing that portion of the "store" area online in the next few weeks.
Also, if you have ideas for shirts - related to the book or to topics I discuss elsewhere, shoot me an email with the design - I'd be happy to incorporate your work into what's available at the store.