I doubt Mullvad has anywhere near the volume of transaction Valve does. And mullvad has plenty of other payment methods, so only a tiny, tiny fraction of their userbase likely pays in mail-in cash.

I don't think Valve could feasibly implement this at their scale - especially if this method was the _only_ way to acquire the games in question.

This realistically doesn’t work that well above anything like a micro scale. It’s also a crime to mail cash across many borders, so it only really works domestically.

What about a system of agents who locally take cash then bulk transfer to Steam? Like some kind of middleman: a processor, if you will, of payments.

Cash handling isn't really the problem with this suggestion