The costs of shipping cash are: $1 or less for a security envelope, $1 or $2 for an international stamp (unless you live in Switzerland in which case it’s less). I usually just use the pen and paper I already have around or scavenge some scrap to write on.
I get the one time code from the mullvad website, write it on a piece of paper, put it and my payment in the envelope, write the address (no return address), apply the stamp, put it in some mailbox and wait for it to hit my account.
It’s often more than two weeks because they don’t process cash and mail every day. That doesn’t bother me because I keep an eye on the months left and re-up with plenty of time left.
Is it worth it? I tend to put more trust in the anonymity of easily broken chain of cash transaction than in the immutable ledger of cryptocurrency technologies.