Knight
He was being sarcastic in that post, it is a probably at least 2 factors higher.
It is more like mailing contraband.
Large sums of money they will catch sometimes. It is easier to send a payment in a birthday card with a couple bills, western unions or a preloaded card. If you send it in the envelope it came with the chances of it get noticed are approaching zero. You can do this anonymously but you to have some sort of real senders address, this is the first criteria used by USPS to flag mail. They can confiscate cash because muh war on drugs so there is a minor risk with cash.