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.