The best way is to pay them and have them sign an agreement that they won't sell the art etc. anywhere else. This is how other card companies work if you want sole ownership of the work.
The other option is to license a design. If you will be using the same design or sentiment multiple times, you could pay them a small amount each time it sells.
In the article, there are examples of both. For example, on Zazzle, they pay a small fee each time a product is sold.
The other card companies pay a one time fee.