They do prevent you from linking to your own store within your Steam game though. Even though they don't provide a complete solution for things like microtransactions and DLC.
How it works on Steam:
- User makes an in-app purchase using the steam wallet integration
- Steam processes the payment taking 30% and gives you a reference number for that transaction
- You query that transaction every time the player logs in to see if they've refunded it or not. That transaction doesn't actually contain any information about what they bought though.
- You then maintain a separate purchasing server whose whole job it is is to keep a record of what the player purchased in reference to that transaction number.
For that Valve wants 30% of in-app/DLC purchases. At that point it's stripe and nothing more. Unlike standalone DLC Or expansions, these unlock purchases don't come with serving any additional content in the form of downloads.
If you make your own service to handle these transactions (with only a 3-4% transaction rate) Valve will prevent you from linking to it, or mentioning it anywhere on your page, forums or within the game itself. You need to direct players elsewhere and then mention it. Even for cross-platform games where having Steam maintain a transaction list for a portion of the users is just a needless additional layer.
But there is always an excuse. Epic tried that. Companies complained.
Their sales used to give you a reusable $10 off coupon. That didn't change the amount the companies got when someone bought their game. It only changed how much they paid. When one of the Witcher games had that coupon applied to it, the developer got pissed off and changed the price of the game so that it was a cent or two below the threshold to activate the coupon, and then fans of the dev were excusing it claiming that they couldn't let the price be lower because it would 'devalue' the game.
if a game was $30 on Steam and $25 on Epic (as a regular price), or some other service, you'd undoubtedly hear the same rhetoric.
Epic's cut is 12% not 30%. They also waive the 5% royalty fee over $1 million for sales on the Epic Store if you use Unreal. Epic doesn't control the prices. Devs set the prices. They leave the price the same on Epic so that they can actually get a little more for each sale.
What the should do on a $60 game though is to set the price at like $56 on Epic, it would encourage people to save a couple bucks there, while still getting them more than steam after the cuts.