Take notice that the location of the customize button is different on shirts than on other products (I missed it at the beginning, and thought some shirts were un-customizable). 

 

There are some products on Zazzle that are un-customizable (less than 1%), however any products you see on this site are.

 

The following is the button location for: stamps, (gift) cards, mugs, pet clothing, posters, shoes, business cards, mousepads, aprons, hats / caps, bags, pins, sticker, magnets and keychains: