It differs from agency to agency. You have to read instructions of particular site carefully. Shutterstock has some restrictions regarding print on demand ("Restrictions: Use or display any Image on websites or in connection with any service designed to sell or induce sales of "print on demand" products using or incorporating Image(s), including, by way of example only, postcards, mugs, t-shirts, posters, giclee prints, wallpaper, artwork and other items.")
On the other hand for example GraphicLeftOvers offers Merchandise License (up to 10000 copies) for 45 USD.