-Indeed a letter of guarantee is available when you send your BTC and you can obviously send it to our support address, unique intermediary for all your questions or possible problems;
mixero.io@protonmail.com.
Having the PGP public key available somewhere on the website (may be on the footer section) liked or allow to download will be a good addition. That way it will be available to everyone to verify the signed message. send from the Letter of Guarantee.