I also thought that receiving cash is 100% safe, until this happened: Spanish "justice", fraud, MITM, convicted because 0.5% of 1000 is 50 - #5 by xmrk .
I would perhaps tweak the message to include the seller id, in this case something from his onion address, so the message would be like “I am buying bitcoins from <insert first 5 characters of peer’s onion address here>” . (Or trade id instead of seller id.) For cash deposits, the btc buyer could be required to hand-write the message with a pen onto the paper bank receipt, and send a photo of this to the seller. I really do not see a downside, banks or post office are not going to see the message.
Also note that I had two other similar scam attempts, both on localbitcoins in 2015 or 2016. Fortunately I was very aware of the risks. In one case the victim sent me money with message about dogs/puppies.
In another case, the scammer put a fake sale ad on ebay, the payment message was some numbers. The interesting thing is that the scammer provided me with bank receipt and he hand-wrote the message … except he did it in Photoshop. Fortunately it was obviously photoshopped, he just put draw a white rectangle over the photo of the receipt, and wrote the message over that rectangle. But if he had been a little more sophisticated, perhaps he could have fooled me.
So perhaps rare, but definitely can and does happen, cannot believe that such scams are not more rampant on bisq.
edit: ok, I see the downside if you send the message with the postal order, but this not applies to sending just the photo of receipt, hope it is possible with USPMO also. Even if selling BTC is illegal, you can simply not reveal the photo you received, so the result is the same as having no such photo.