HoboNinja offers the A Very Merry Star Wars Ugly Christmas Sweater Shirt for $9.99. Coupon code "THEMERRYSIDE" cuts that to $7.99. With free shipping, that's $12 under the lowest total price we could find for a similar shirt. It comes in sizes S to XL.
Note that size XXL is available for $1.89 more, and 3XL for an additional $2.49. Also, add two regular-price shirts to cart and apply the above coupon for a final price of $13.98 with free shipping. (This way, if you ruin one, you can say "No... there is another.")