I’m with GL on the actual production of the merchandise but I’d just like to point out that people can get their own merchandise shops with certain sites. All you do it upload your images and then you can place them on any type of product.
You don’t even have to buy stock as it’s just a virtual shop, so you direct people to the site and if they buy something the company who run the website print the image on to the desired product and send it out.
Does that make any sense?