Methods and apparatus for on-demand customization of products. A product distributor may provide a product customization service and user interface to its customers that may be used to enhance, extend, customize, or combine stock physical products offered by the product distributor by providing custom add-on physical items for or customizations to the stock products. The product distributor may leverage on-demand production technologies such as additive manufacturing technologies to produce the custom physical items or customizations according to specifications received from the customer or generated according to descriptive information for the stock products. The custom items may be combined with one or more stock products to produce customized products for the customers.