There's something I lost when I switched from painting to digital art, and that's the original work.
You can't buy the one, single, original piece that I create any more, because there isn't one. So the very first merchandise I wanted to sell online was posters, and archival art prints, made from my high resolution digital works. Everything else - the t-shirts, books, coffee mugs, and so on - has grown from that one desire.
Orders for merchandise from 'Ars Celtica' are filled by Zazzle. They're billed and shipped separately from merchandise ordered through the vendors who supply 'Saga Shirts' and my other products.