See my previous replies. You can insure the cash and the seller of Monero has to put up 115% at least of the value of the trade. The buyer at least 15%, if either tries to scam they lose it all.

Registered mail provides a legally binding chain of custody, you not only would it be difficult to convince and arbtrator that the other party didn't actually sign for the delivery or didn't send the cash, you would also be facing felony fraud charges.