Alternatively, you can set up microtransactions on a website using the Steam WebAPI and open that from the game.
You mean like opening a webpage in an iframe inside the game window? I guess I will also have to make some API calls from the game to retrieve the status of purchases?
If I choose to sell a DLC pack (say, new levels for the game, just a simple text file), how will my game recognize that the DLC is purchased? Will it be installed in the same directory? Can I simply check if the filename exists in the home directory?
Sorry if my questions are silly, I have zero experience with all these..