Hi,
have you ever bought BTC or other crypto money paying by cash banknotes sent by mail?
I think it's cheap and also the only real anonymous way to buy crypto or any other item, but post employees could open the envelope!
Is there some safe way to hide the banknotes inside the envelope?
First, it is best to buy in person (when available). Mailing money has its risks. There is always the risk that the letter could be lost or opened and the money will be seized. There is the risk that the Bitcoin seller will abscond with your money (even with escrow, he can deny he received the letter or that the envelope had no money in it).
Second, if you cannot buy in person, it is best to make a bank deposit. You will have the receipt for proof. So if the bitcoins are under escrow, you can always prove you deposited the money a bank account.
Both methods are anonymous.
If you are going to mail money, then use a security envelope with printed lining (the contents will not show). But, yeah, it is a good idea to hide the money in an extra layer of paper anyway. Don't overdo the paper, otherwise this may raise flags at the post office.