Mullvad will accept envelopes of cash. They go far and above any other company when it comes to helping their customers maintain anonymity.
people really like to mention the cash envelope, whoever came up with it is a genius
And yes, whoever invented cash was a genius. It's an amazing technology that, in some ways, is far ahead of almost all digital payment systems.
OTOH, I did the cash envelope thing only once, out of curiosity. It was fun but now, for convenience, I just buy regular vouchers online.