Aria here, one of the Social Care gurus at Shopify.
I do believe that is the only one. There are a lot of Print on Demand companies in our app store but although they sell many product types, greetings cards don't seem to be a popular option.
I'm excited to see what sort of product you come up with. Will, it just be greetings cards or do you plan on selling other types of products? Do you have ideas for your marketing plan? If you haven't already, check out our blog for creative marketing ideas. We also have a great Instagram Marketing course on Shopify Academy.
Let me know if you need anything else!
Social Care Guru