LocalCryptos, while not having a specific cash by mail option, do have a "Other" option where people can specify their own payment method, so might be worth a look.
And as I said above, if you do not find an offer you like, then post your own. If I had a satoshi for everyone I've seen who wants to use a peer to peer exchange, but just gives up after not finding an offer they like, I'd be a rich man. You might find that some of the cash in person traders would be happy to trade cash by mail if they saw an attractive offer.