Buying your designs could be written off as a business expense if you gave them away to other people and asked them to send people who like the shirts to Amazon. Then it becomes a promotion. If you simply kept the shirts yourself, it's updating your wardrobe. Of course if you had the poor fortune to be audited, you could tell the IRS you gave the shirts away as a promotion whether you actually did or did not. I doubt they will show up at your home and raid your closet! A business expense of a $100 or so dollars shouldn't raise any red flags.