Don't know how that could be accomplished. I would think we're talking about "cash only" employment. You have to give up a SS number to open a bank account. While cash is not likely to be sent through the mail there's really no way of determining what the source of money is when it's purchased with cash. Sending money orders is not a crime I don't think. Neither the bank nor the PO has a basis for asking questions about earned money. And on top of that, "personnel" questions are considered confidential information.