http://store.steampowered.com/itemstore/283880/
When HQ was first released on Steam, our intention was to include Steam trading cards in addition to achievements, leaderboards, and several other features that add to the game. Unfortunately, Valve's policy does not allow free games to give out trading cards with gameplay. We considered handing out cards to players high on the leaderboards, but it turns out this doesn't work under policy, either.
Trading cards are very popular on Steam, and therefore, one of the most frequently asked questions on this forum has been how to get cards for HQ. For players who want to craft a Heroine Badge or show off these cards on their profile, we have two options available.
First, if you have any Steam cards you don't want (e.g. because they're duplicates) you can convert them into 'gems' and you can use these gems to craft booster packs. Craft a HQ booster pack to gain a set of trading cards. This has worked for several years now. Not all players are aware this option exists, but at least one Steam player has a level-5 HQ badge by now.
Second, Steam does allow trading cards with in-game transactions. We have set up an item store for this purpose. You can buy a decoration for your favorite character class, and after about ten minutes of playing you'll get a trading card drop. You can use this to complete your set or trade them with other players. The link is at the top of this post.
Please note that the minimum transaction required for trading cards is set by Valve, not by us. Also please note that these items do not affect gameplay and are not required to complete the game. They're simply an extra option for players who love trading cards. Enjoy!
