In my game which I’m going to put up on Greenlight later, I want to be able to have Steam features such as achievements and trading cards.
I’ve seen there’s a $50 asset package called ‘Steamworks’ which lets you do that however I’m unsure whether it covers the latest trading cards and I don’t want to spend a single cent.
How would I go about doing that without spending money (other than buying Greenlight license)?
Thanks!
Try Ludosity’s Steamworks Wrapper.
It doesn’t specify “Trading” however. I didn’t use it since I don’t have an AppID yet, so I can’t say for sure, but it says the package is free so I don’t think you have anything to lose. Please note you need the SW SDK for this, obviously 
There is also Steamworks.NET which has Unity package under its install page http://steamworks.github.io/
I took a look at Ludosity’s Steamworks Wrapper as well but wasn’t sure it was still supported so dug around for alternatives. The Steamworks.NET seems to be actively used as its Issues forum shows activity recently.