I do indeed! I’ve sent USD, GBP, and Euros from all over the world - Europe, North Africa, South America, and the US. (I was a “digital nomad” pre-pandemic.) They even give you a one-time-use token for cash payments so you can maintain the anonymity of your account number if the letter is intercepted.

Other than the letter I sent from Morocco, I never included a return address.

That is one of they many ways in which you can add money to a Mullvad account. You should try creating an account; it only takes one click.

I just signed up with Surfshark but I'll definitely check out Mullvad.

When he says it's just one click to generate an account, he means it.

https://mullvad.net/en/account/create/

That link automatically creates a new account. No names, no email address (except if you set up a PayPal subscription), not even a password. Your account number is literally all you need. They even have a FAQ that provides a generalized database schema.

https://mullvad.net/en/help/no-logging-data-policy/#numbered

It is possible (and yes, I tried it!), although other payment methods are available, including some more conventional methods.