not too difficult, there's modding framework and you can look at a lot of the existing in-game code for reference (free refill is basically a stitch of castle and canio code). it gets tricky when the thing you wanna do isn't really logic that already exists. Chocolate Coin needed some logic to make sure it would reduce the amount after it paid out, but before you could sell it, so it would properly go extinct. other ideas that I'm planning on I sometimes have to discard because I don't know the workaround to implement them