Japan lets you make payments for online content at convenience stores.

How it works is you purchase a product online and it gives you a barcode that can be scanned at any major convenience store. You go to the store, scan the code, hand over cash, and the content you bought is instantly unlocked once the payment is confirmed.

Steam sells physical gift cards. You can buy them at convenience stores, Walmart, etc. you can pay cash for them.

those stores would absolutely stop carrying the gift cards if customers could not pay with visa/mastercard for them.

Mullvad VPN takes cash, you post it to them.

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

I would say monero but it is actually superior because there is no serial number tracking.

The closest thing is via an instant bank transfer, like the new FedNow protocol in the US.