To do this, it uses a new in-game purchasing system built off of the in-software Web browser. The first title to feature this is The Maw, which now includes two additional levels that can be purchased for $1.25 each, then played immediately. Previously, all add-on content was sold as a separate purchase from Steam's game store.
The new feature is available to all developers as part of Valve's Steamworks publishing platform. For games with existing add-on content, this means that the companies will be able go back and update their titles to allow in-game purchases.
Micropayments are becoming an increasingly important part of modern games, not only as a way for publishers to continue to make money from a title after it's released, but also to help bring in extra sales from people who purchase games used.
Valve says the new system will allow users to purchase and add additional content, even on titles that were not purchased through Steam's game store, just like it does with physical software titles that were purchased from other retailers.