Defying Gravity and the Law of Supply

There is probably some elasticity rule or other factor of supply curves that I haven't realized yet but I may have found something defies the law of supply.

Supply curves show that as price increases, there is more production and more quantity supplied. Supply curves slope up due to the law of diminishing returns - its harder to make more/cheaper goods after the first set of goods.

However, I had my OBOC (Off Broadway On Campus) show this weekend and we ordered shirts with a 4 colored print of "Defying Gravity" (a Wicked song). The shirts cost each person $17 and costed the producer some price to make. But here's the thing with tshirt prints. It costs MORE to make that first tshirt. After the print is made for that first shirt, the price decreases for each shirt produced - it's easier to make tshirts. This defies the law of diminishing returns.

So when the quantity supplied increases, the price actually decreases for the producer. This doesn't follow the Law Of Supply!

Again I may be missing some factor or law of elasticity, but do printed tshirts defy the law of supply and diminishing returns?