Scirra does provide a Greenworks plugin, but it doesn't work very well, theoretically you can only unlock achievements and activate the overlay with the Greenworks plugin, so there's no way to access the leaderboards for example. But the worst part is the support for it, since there are really few people working with C3 and Steam, Scirra doesn't seems to be very interested in supporting anything related, the Greenworks plugin doesn't even work with the latest version of NW.js, so you will have to re upload your game with an older version of NW.js, which can cause some compatibility issues. The overlay doesn't even work at all without some extra command lines that you will have to find out by yourself, since they're not mentioned in the documentation. Well, you can already tell that i'm very frustrated with Scirra support for Steam haha, but i'm hopeful that they will fix everything like they always does. In C2 you had to pay for a third party plugin, but at least it worked well.
Cards are easy to setup, you basically just need to upload a few images to Steam and they’re done, unfortunately Steam blocked the possibility to release a game with cards, now you have to wait an undertaminated amount of time before they allow you to add cards to your game, because some developers were exploiting the card system to make some easy money, which is sad.
Steam isn’t very bureaucratic, i didn’t needed to open a company or anything, and they literally send the money directly to your bank account, no PayPal or anything like that.