Delta Rune shirt: Let people know you're qualified to help them through the ruins. Comes with a sticker, too!
Mercy shirt: You wiggle your hips. The t-shirt wiggles, too. Also include the 'Flowey' pin.
Smooch-Cut Undertale Stickers: A full sheet of kiss-cut Undertale stickers. Put them next to your dialogue box, so that people know exactly what you mean when you're talking.