Once or twice, but only ever with another party whom I have traded with several times before and built up some trust and rapport with.

The main problems with cash via mail are privacy and security. The bitcoin seller must obviously give an address, and from our discussions regarding how to anonymously receive a hardware wallet, that can be difficult to do without compromising your privacy. However, if you do have a way to anonymously receive packages, then this can be an extremely private way to sell, and can be completely anonymous for the bitcoin buyer.

There is also the security of sending large amounts of cash through the mail, and how to verify that neither party is attempting to scam the other. Bisq have a great tutorial on how to do it as safely as possible here: https://bisq.wiki/Cash_by_mail. I still probably wouldn't pick it as a method with a first time trading partner, though.