Depends on the evidence. With registered mail, the recipient has to sign for the package and the sender has to package the cash in the presence of a postal worker and declare the value truthfully under threat of fraud charges. Then there is a legally binding chain of custody from each postal worker all the way to the recipient.

This isn't unique to Monero, Retoswap is a fork from Bisq which was the original p2p Bitcoin only exchange. Cash by mail has been a popular payment method there for years before Retoswap and it functions the same way with the security deposit + arbitrator system.