The short and easy answer is: no, I don't require any payment or licensing for your use of the cards, even physical prints for commercial use. Good luck with your game!
The more involved and difficult answer is that I don't own most of the built-in art, and I don't technically have the ability to transfer the rights to use the card images to someone else. This means that some of built-in card frames and icon art may be covered under licenses that require attribution or require separate licenses if you want to use them commercially. This differs on a case-by-case basis for each asset, so if you're concerned about this and would like a link to the original art asset for any particular cards or icons, please let me know.
