They run a little pricy for hats, at $24.99, but each hat is custom made to order -- you get your guild logo (with all of the game's choices and colors, or pulled straight from the Armory if you want) on the front, and a Horde or Alliance icon on the back, as well as your guild or character names if you'd like. Hats are flexfit and come in two sizes: Small/Medium or Large/XLarge. Paladins, you'll likely need the larger version.
Pretty smooth -- I like the fact that it's just the tabard logo. The shirts can be a little overboard with all of the tabard art and the text and the class icons, so if you want to show some guild pride a little more subtly, this seems like a great way to go.