During mid-2011, Valve began to offer free-to-play games, such as Global Agenda, Spiral Knights and Champions Online; this offer was linked to the company's move to make Team Fortress 2 a free-to-play title. Valve included support via Steamworks for microtransactions for in-game items in these games through Steam's purchasing channels, in a similar manner to the in-game store for Team Fortress 2. Later that year, Valve added the ability to trade in-game items and "unopened" game gifts between users. Steam Coupons, which was introduced in December 2011, provides single-use coupons that provide a discount to the cost of items. Steam Coupons can be provided to users by developers and publishers; users can trade these coupons between friends in a similar fashion to gifts and in-game items. In May 2015, GameStop began selling Steam Wallet cards. Steam Market, a feature introduced in beta in December 2012 that would allow users to sell virtual items to others via Steam Wallet funds, further extended the idea. Valve levies a transaction fee of 15% on such sales and game publishers that use Steam Market pay a transaction fee. For example, Team Fortress 2the first game supported at the beta phaseincurred both fees. Full support for other games was expected to be available in early 2013. In April 2013, Valve added subscription-based game support to Steam; the first game to use this service was Darkfall Unholy Wars.
