As someone else in the thread correctly mentioned, purchases and payments are made through Steam’s platform, which means these things would have to be restrictions would have to be implemented by Valve through the Steam account and payment management side. This isn’t a limitation they currently have in place, and it would also introduce a new issue which would need a new solution – cards of customers expiring, getting lost or stolen, or accounts closing or switching. Because payment is made through Steam, as previously mentioned, this would require additional Valve support resources to handle as well. Unfortunately since we aren’t a part of Valve, it’s not really something we can sign them up for to make happen.