It looks like those of you planning to purchase a few of Welovefine's Rarity shirts in the future have a bit of a bonus in store. The unicorn horn up above is included with any of the following:
Rarity Pop Art, Rarity's Boutique, Magnificent Rarity, Rarity Cutie Mark and Rarity It Is On
It doesn't look like any kind of money amount is required, the horn is just straight up included this time around!
Images of all the shirts can be found after the below.