There is also the Steamworks Web API which allows you to make HTTP calls to do things like:
- Set or retrieve a user's stats, achievements or leaderboard scores
- Execute an in-game purchase (using Steam Wallet)
That might be an option rather than using the Steamworks C++ API, although they are protected methods that require authentication and are intended to be accessed from trusted back-end applications.
There are at least 5 Construct 2 games released on Steam, see here
In-game purchases for games on Steam would most likely require the use of the Steam Wallet, which you need to access via the Steamworks API (either Web or C++).