Marketing and promoting the business, creating new designs, sourcing products, and administration are the other aspects of running a business that will eat into the time available. You will need to allow for over half your time to be spent on the business side of things. You will also need to think about how you will handle the business if you decide to take a vacation, if you get ill, or managing extremely busy periods. This is why good planning is essential. It will help you avoid any nasty surprises later on.
Think about the product that you are selling. Put together a product development plan that shows how versatile your idea is. Can it grow? Does that product spawn other products or idea that can potentially sell? While simply making some cards and putting them up for sale might prove successful, you will likely need more than the initial product. A product development plan will help take out the guesswork of how to make products that sell.